For starters, there are sites which have folded, some new competitors, and sites that changed their model significantly. At the same time, there has been pushback from airlines and suppliers, some of which have resisted the aggregator model. (The lawsuits between American Airlines and Kayak, which initially resulted in American Airlines no longer being listed in Kayak results, was perhaps the most prominent case of pushback. Since October 2008, aa.com results are back in the results. More on that below.)

The result: The golden ring of a truly complete search, covering all the options and all the providers, is still a ways away. No single site actually finds every flight option, every fare, or every seller.

But that doesn’t mean that there aren’t differences between the aggregators. It’s time to disaggregate the aggregators again.

This year, each site was put through multiple tests. Four kinds of itinerary were tested: A large-city to medium-city domestic US flight with multiple carriers offering direct service; a medium-city to small-city domestic US flight with at least one change of plane required; an international flight with a US origin; and international flights (from Paris to Dubai, and Manchester to Madrid) to test how sites do for non-US flights. For each of these flights, I tested a short-term booking (7 days advance purchase) and a longer-term booking (30 days advance purchase).

This time, I compared Kayak, Sidestep, Mobissimo, TripAdvisor Flights, Momondo, Skyscanner, WeGo (formerly Bezurk), Trax, Farecast, Fly.com, and Dohop. Sites which were on the list last time but either folded or stopped doing metasearch include FareChase (bought by Yahoo, then abandoned in March 2009), PriceGrabber, and Qixo.

So which aggregator came out on top in 2009? Here’s the summary, with site-by-site reviews thereafter…

• Overall best bet: TripAdvisor Flights. Low fares, good features, a solid performer overall. It took them a while to join the game, but it’s a strong product with a small but important edge over its closest competitor, Kayak.
• Lowest price, domestic: If you’re searching for domestic US airfare, most of the aggregators did pretty well, but the lowest fares were consistently on TripAdvisor Flights, Mobissimo, Momondo, or Trax. (Kayak and Sidestep would have been included in this list, but they often offered the lowest flights at the same base fares, but typically directed you to Orbitz, instead of to the airline or to a no-fee site like Priceline, to buy the ticket. Orbitz still charges a booking fee as of this writing, so single-airline itineraries can typically be purchased for less elsewhere.)
• Lowest price, international from US: If you’re searching for international flights starting in the US, try Kayak and Trax.
• Lowest price, international, ex-US: If you’re search for international flights that don’t originate or end in the United States, Mobissimo and Momondo performed best.
• Class conscious: If you’re looking for a deal on premium cabin fares, fly.com is your site.
• Sorting features: If you want control over options, and like to tweak and limit your results, Kayak gives you the most power.
• Real total cost: TripAdvisor Flights lets you estimate what your actual costs will be, including luggage checking fees, headphone charges, etc.
• Avoid: WeGo, which offered impressively-low search results, but none were actually bookable. Also avoid Dohop, whose prices were consistently higher than their competitors.

Individual site reviews

TripAdvisor Flights
Although a clone of its competitors in many respects, TripAdvisor’s new aggregator offers one big difference: The site offers an analysis of how much the flight will actually cost you, once you figure in the baggage fees, the headphone fees, etc. It even considers what your elite status is. That’s a great feature. Second, and more disappointingly, the site (unsurprisingly) favors its sister sites Expedia and Hotwire in its results. But those sites add no booking fee right now, so no big whoop. The site combines the best sorting features of Kayak with the added value of fee analysis. TripAdvisor takes it by a nose.

Kayak
Kayak is the largest of the metasearch sites, and it offers solid results. But it didn’t consistently offer the cheapest fare, because the universe of sites it searches is sometimes limited by agreements with individual partners. (American Airlines, for example, won’t allow results from its website to be shown alongside AA flights sold through Orbitz.) Orbitz puts demands on its partners, too, and subsequently, Kayak is heavily Orbitz-centric, with no representation of Expedia, Travelocity, or Priceline. Actual fare availability was good when I clicked through to sellers’ sites. Kayak’s sorting features are still the best (so good that they’re copied wholesale by TripAdvisor), with the ability to narrow search results by time, airline, or aircraft type (e.g., no props or RJs), to name a few. The site has recently added “flight quality warnings,” such as on-time percentages and tight-connection alerts, but those aren’t a big deal.

Sidestep
Kayak bought Sidestep a few years ago, but continues to operate the site as a standalone entity. Frankly, I don’t see why. The results are identical, the search tools are nearly the same, and there’s nothing to recommend this site over its parent. Everything stated above is true here as well, but why bother?

Farecast
Farecast, now owned by Microsoft, is most famous for their fare prediction engine, which suggests whether ticket prices for your route will drop in the future. The predictions are pretty accurate, though they don’t account for times of day, routing, or airline, if you have preferences on those matters. (And you should.) Farecast also offers up an aggregator, albeit a fairly minimalist one with few options and a favoritism for Orbitz results. Come here for the fare predictions, but for live fare comparisons, you’re better off elsewhere.

Mobissimo
Mobissimo claims to search more sites than anyone else, but not necessarily for a domestic US search. They have good coverage of Asian and European airlines, but their controls and filters are less than others’. Decent results, but I preferred others’ display of information.

Fly.com
From the folks who brought you TravelZoo, Fly.com offers one gimmick that’s worth noticing: First- and business class fare comparison. That’s it. Otherwise, it’s a clone of other sites. If you’re not looking for premium cabin seats, skip it.

WeGo (formerly Bezurk)
Big on teasers, low on success. WeGo touts some amazing fares, but good luck buying them. They also don’t list the lowest flight first: They show a sponsored link at the top. Bad, bad, bad.
Update April 9, 2009: WeGo has eliminated the sponsored results feature.

Skyscanner
Britain’s SkyScanner offers decent results for UK-based searches, but it’s not as helpful to others. For US and non-UK searches, SkyScanner searches returned far higher fares than the average.

Momondo
This Danish site is quite good at ferreting out obscure sellers of travel, especially for intra-European itineraries. But be aware that you’ll often get results from suppliers in countries outside the destination or origin. Flying Chicago to Baltimore? Your ticket could be sold by a British company (which could mean a 3% credit card surcharge).

Trax
Trax.com offers a Kayak-lite search that works well for domestic US itineraries. They include Priceline and Cheapoair, which others don’t, but the results weren’t significantly better.

Two other sites of note:
1. An aggregator you can’t use for booking, but which is great for research, remains ITA Software. Their engine powers Orbitz, and backs up Kayak and TripAdvisor results. They’re a great resource. (Click “log in as a guest” to use the search without registering.)

2. No discussion of fare search would be complete without a mention of FareCompare. The site’s alerts give you a heads-up on future sales by notifying you when fares drop, before they go on sale. They offer fare search, too, which is powered by Kayak.

So… what’s your experience with fare aggregators? Hit the comments to share your stories.

I would like to find an airfare search engine where I can put in an airport and see:

a) Where all the direct flights go and the prices for each.
b) specify “Europe” or “Asia” and see the best prices to countries in those regions.

Why you ask?
We frequently travel last minute, and frequently we do not care where we go, we just like to go. So if we have 5 days with nothing to do and want to go someplace, anyplace, in Europe it would be handy to see what the lowest price option is.

First off, I admire your flexibility, AJ. Way to go.

Your first criterion, finding only the nonstop flights from a particular departure point, is tough. Some search engines will let you specify nonstops only when you’re searching specific dates, but I can’t find a nonstop limitation on any of the broad, flexible searches I’m familiar with. (Other readers are invited to chime in with suggestions in comments!)

As for looking for the cheapest flight for ultra-flexible destinations, you’re in luck:
The two best options right now are offered by FareCompare and Mobissimo. Travelocity offers an option for domestic travel. ITA Software has a solution, too, but it requires more work and is not as flexible on dates. Here’s the breakdown:

- FareCompare offers a flexible destination search through their “Destination Deal Maps” in the middle of the page. Click on the continent you want, and a list of fares will appear. Clicking on a fare shows you the dates eligible for the fare. Pick a date, then an airline, and the system checks seat availability. FareCompare doesn’t sell tickets, so you’re directed to one of the major online agencies to close the deal.

- Airfare aggregator Mobissimo also offers a search like this, bizarrely located in the “activity search” tab. After selecting your departure point, you can select the desired continent from the pulldown. It’s odd to see “Europe” or “Africa/Middle East” listed as an “activity” right alongside “beaches,” “gambling,” or “opera houses.” But hey, the search works. Like FareCompare, Mobissimo doesn’t sell tickets, but directs you to the seller.

- Both FareCompare and Mobissimo effectively mirror Travelocity’s Dream Maps in format. But Travelocity recently neutered this tool for international travel searches. It still works well for domestic searches, and the site sells tickets directly. (The flexible international search was taken down because the fares didn’t include the fuel surcharges. See here for an explanation.)

- One final idea would be to use ITA Software’s search. ITA’s search requires you to input destinations, but it allows you to string a number of options together. For example, you could enter your departure city, then add a boatload of contending city names or codes to the destination field in the form. Say you’re interested in going to Europe, you could enter something like “ams;fra;par;lhr;dub;mad;cph;ath;rom” — a string of European cities’ airport codes, separated by semicolons. Then widen the destination search by using the pulldown menu to include any airport within 300 miles. Then click “more options” and uncheck “allow airport changes,” to make sure you arrive and depart the same city. That will pull in a LOT of destinations for the dates you want. ITA doesn’t sell tickets, and doesn’t point you to a seller — take your pick.

Note that I went the next step and also reviewed aggregator hotel searches… Let’s see if they follow suit and do the same!

(snideness: off)

AP’s Jesdanun agrees with my assessment of Kayak as the superior airfare search, but BusinessWeek’s Lacy downplays Kayak and instead favors SideStep for its downloadable toolbar “big brother” feature. This is a reason NOT to like SideStep if you ask me. I just don’t trust these browse-along features to protect my privacy. Call me paranoid.

In aggregator news, Mobissimo has in recent weeks made some improvements, broadening the number of sites they search (and pitching the means by which they conduct their searches.) Their recently launched India-based site is a big move. But I still just don’t like their user interface, their comparative lack of information, or their lack of controls. It’s getting better, but it’s not there yet.

Two weeks ago, I offered this assessment of airfare aggregators — sites that perform an identical search across multiple travel sites and report the results to you. But most of these sites can be used for hotel searches as well. How do the aggregators stack up for lodging?

Hotel searches are trickier than airfares, for the most part, because not all rooms are created equal, and hotel location is less standardized than air travel routes. Finding a low rate isn’t helpful if you have strong preferences for two queen beds vs. one king bed, or smoking vs nonsmoking, or if you need to be within walking distance of a particular location.

(Note also: For many large chains’ loyalty programs, you now are required to reserve the room through their own sites in order to collect the points. The aggregators may be helpful in finding a lower rate booked through another site, but be sure you’re aware of point accrual rules before committing to anything.)

In contention here are Kayak, SideStep, FareChase, Mobissimo, Qixo, Bezurk, PriceGrabber, and the newly-launched hotel-only SearchParty.

I ran three separate searches on all engines: one stay in San Francisco, one in Paris, and one in Moab, Utah. The idea was to test large and small U.S. destinations, as well as an international location. I also ran the same searches on Travelocity and Orbitz to see how the traditional sites compared.

Once again, Kayak comes out on top. But this time the competition is closer on its heels. Farechase comes in a close second for North American searches, with Sidestep and Bezurk coming close for international. Worst performer of the bunch, by far, is Qixo — when its servers were working at all. SearchParty is unique in showing the cancellation policies, but its prices are still too unreliable for me to recommend the site; in time, it may improve.

The breakdown:

Kayak
Kayak offered a comprehensive mix of independent and chain hotels, including hotels you couldn’t book online. Their goal — to catalog every hotel on earth — is not quite reached, but they do an admirable job. They even include small, independent hotels that can’t be booked anywhere online (they give you a phone number in such cases, but these offline hotels may be listed without rates). Kayak’s controls are also the best. The difference: More options to let you narrow the search, and not just sort results. For example: While others, such as Farechase and Sidestep, let you narrow down results by neighborhood, and Farechase lets you sort results by distance from a given point, only Kayak lets you narrow the search by a geographic radius — without sacrificing the other variables you want to control. It’s a subtle difference, but this level of output control makes filtered results far more relevant. They include a small map as well (powered by Google), which is helpful. Downsides: No reviews or photos (minor quibbles), and they could offer more room amenity options to filter results.

Farechase
Update: Yahoo has abandoned Farechase; this review remains up for posterity, but the site is gone.
Yahoo’s Farechase responded more quickly than the other sites — the fastest response in the group. Controls to narrow results include price range, class (stars), neighborhoods, bed type, and an amenity list. The results can be sorted by popularity (default), price, name, or distance from the search term. Results include a small photo of the hotel, if available, but I’m really not sure how useful this is. Sometimes the quality is poor, sometimes it’s just a photo of a bed. You can’t garner much from the photos. The map is huge — maybe too big, even. One nice feature is the consumer reviews, garnered from Yahoo and Travelocity. TripAdvisor is still a far richer resource for reviews, but having user reviews is a positive, nonetheless. Major downside to FareChase: Searches are possible for US and Canada hotels only. Get a passport, guys!

SideStep
Sidestep’s output is similar to Farechase, but without a map, and without a way to limit bed types. Like Kayak and Farechase, sponsored results get top ranking. Photos are the same size and quality as Farechase. A small number of Sidestep-user-generated reviews. Prices were good for international searches. A solid performer.

Mobissimo
Mobissimo has fewer controls than the others. Default ranking is by price, without regard for location or class. You can sort results by stars, price, name, or provider, but it doesn’t give you any choices over the location of the hotel or the kind of room you’re searching. International search results were good, but the results are so hard to wade through, it’s still an inferior search.

Qixo
Like Mobissimo, Qixo doesn’t offer many controls over the output. For some reason, the site had trouble loading on one computer I was using, but worked fine on another computer in the same network. (Qixo shifts from qixo.com to res99.com for search results, which may have something to do with it.) But regardless of the networking issues, it was still an inferior product. The highest prices, by far. The least control over the output. Don’t bother with this site.

Bezurk
Bezurk found the lowest prices for the Paris search, but it was far less effective for US-based searches. (Their best deals seem to come from HotelClub.com, which I did not see listed in any of the other aggregators.) Not as many controls as the others, but the deals are good.

Pricegrabber
Update: PriceGrabber has thrown in the towel, shutting down their travel search feature. The rest of their site is still up and running. This review stays up, though the travel service is defunct.
PriceGrabber grabbed some pretty high prices in Paris and San Francisco. Its rates were par for the course in Moab. Their initial search has limited options, but the results can be filtered further (star rating, price, features, etc., but not distance from a particular point). Still, other sites offer better controls. PriceGrabber isn’t bad, but you can do better.

SearchParty
SearchParty, a new entrant specializing entirely in hotel searches, needs to work some kinks out. The site is attractive in its presentation: For each hotel, they show the prices of the three least expensive providers. When you click through to compare the booking options, you see the cancellation policies and fees, which is a VERY nice feature. But the prices that come up in the search are simply unreliable. Click on that $69 rate at the Sleep Inn, and the very next screen tells you it’s actually $99. What good is an aggregator that can’t get its prices right? Further, its searches are limited to the United States only. Finally, there’s no way to limit searches by bed/room type. SearchParty has potential, but it has a way to go.

The “Control Group” in the experiment: Neither Orbitz nor Travelocity found as many options as the aggregators. Orbitz was easier to narrow down and control. But price-wise, the aggregators found lower base rates.

(image: Intercontinental Amstel Hotel, Amsterdam, photo via larscapes.com)

The great benefit of these sites is that you get greater transparency of fares. However, not all aggregators are created equal. I put a few to the test.

For the time being, I limited my comparison to airfare searches. I looked for accuracy (did the quoted price match the price actually offered at the provider’s page?), depth of information (does the site give the booking class, cancellation policies, etc.?), and control (can you sort searches easily, by provider, by price, by distance, etc.?)

For airfare, I compared Kayak, Farechase, Mobissimo, Bezurk, Farecompare, Sidestep, and Pricegrabber. (Since I’m based in the United States, these results may be biased toward North American searches.)

The result: Kayak came out on top, with the lowest prices, the most control over the output, and most information about both airlines and sellers. Sidestep comes close. Farecompare gets an honorable mention for its price-driven approach. All sites accurately reported fares — there were no surprises when clicking through to the target site. However, no single aggregator actually found every flight option or every major travel site.

Kayak
If you know your dates of travel, Kayak offers the most powerful site, in my opinion. It covers a range of websites, including a number of consolidators. The fare results can be sorted by airline, by time, by price, by airports (it searches alternate airports automatically), and by stops. One of the biggest benefits is the ability to see the precise fare booking code, by clicking “details.” (This is great if you’re looking for a cheap but upgradable fare, for example.) A downside to Kayak is that they seem to exclude the big three online agencies — Travelocity, Expedia, and Orbitz don’t seem to come up in searches. On the plus side, JetBlue, who (like Southwest) doesn’t show up in the big three’s searches, comes up for comparison on Kayak. Negative is that they don’t seem to grab every possible routing from every airline (a common complaint for all aggregators). Kayak is 95% there, but not quite 100%.

SideStep
Sidestep is a very close runner-up to Kayak on the pure-airfare search. It covers a similar range of sites, plus includes Orbitz in the search. It has one interesting benefit: offering air and hotel package searches across multiple sites. Note that Sidestep is perhaps best known for its downloadable toolbar, which “watches” where you browse, and offers fare alternatives. I am not a fan of this over-the-shoulder co-browsing, but you don’t have to use their toolbar in order to run a search.

Farechase
Farechase, owned by Yahoo, copies much of Kayak’s template, but searches a slightly different universe of sites, including both Orbitz and Cheaptickets. The total number of sites searched is smaller, but doesn’t overlap entirely with Kayak or Sidestep. One downside: the flight details do not include the booking class/fare code. Presumably you need to go through the process of a complete booking in order to see that info.

Mobissimo
Like Farechase, Mobissimo DOES include some of the online agencies: Orbitz, CheapTickets, and Opodo, for example. However, it offers less flexibility in sorting the data than Farechase, and it doesn’t give much in the way of flight details. While the fares it finds are comparable to Farechase, the presentation is previous-generation.

Pricegrabber
Update: PriceGrabber has thrown in the towel, shutting down their travel search feature. The rest of their site is still up and running. This review stays up, though the travel service is defunct.
Pricegrabber is a comparison shopping site, and their travel search is just one among many. The search engine again follows the Kayak template, but it doesn’t tell you up front which sites it has searched. Results can be sorted by price, airline, time ranges, and, interestingly, ontime statistics. But you can’t see the flights’ booking class here either, the range of alternate airports is limited, and you don’t even know who the seller is until you choose the flight. They do work with Orbitz, and perhaps others, but they need to provide more information.

Bezurk
This is an Asia-based site that taps into a completely different pool of providers. It copies the Kayak model, again. However, because the search is based on Asian companies, it may be difficult to find a fare you can actually purchase if your travels don’t touch Asia. Nonetheless, for international travel, check it out.

Farecompare
This recently unveiled site is notable for its price-driven approach. Instead of entering cities/dates and comparing options thereafter, Farecompare asks for cities only, and drills down on the basis of price. Much like Travelocity’s Dream Maps or Search by Price, you may end up with a great price… on dates you can’t use. However, the site is interesting for the sheer volume of information it provides, and it offers historical data tracking the city pair’s fare trend over time.

What if you just care about price, without regard to anything else? Who has the best price? I did two searches, one for Chicago to Los Angeles, one for San Francisco to Sydney. The city pairs made no difference: In both cases, Kayak, SideStep, and Mobissimo found the identical lowest prices. Farechase’s “lowest price” was more than the others.

Aggregators are a great tool, but even then, you may want to run one or two of them, to see if they differ. Maybe we need an aggregator of aggregators (perhaps metakayak.com?) to truly get thorough searches.

In two weeks, I’ll test the aggregators’ searches for hotels, which offer an entirely different set of challenges. If there are any sites that you believe I have missed, or if you think my assessment is way off base, let me know by leaving a comment or using the contact link at the top right of the page.

Update (April 24, 2006): One reader wrote, reminding me of ITA Software’s excellent fare search tool. ITA powers Orbitz.com for airfare searches, though Orbitz’s search engine is a dumbed-down version with far fewer features. The genius of ITA is that it is incredibly powerful, if you know how to phrase your searches. However, it’s not a booking site, just an informative flight search, so even if you find a great fare, you have to find and book it elsewhere. Since ITA doesn’t actually get you to a booking (and, as the aggregators demonstrate, fares aren’t always available everywhere), I didn’t originally include them in the earlier discussion.

Update (April 28, 2006): Reader Todd points out that I forgot to include Qixo.com in my review. He’s right. Ahem:

Qixo.com
Qixo came up short. It offered less information (fare booking classes, provider, etc.) and had the highest price for identical searches. I ran fresh searches for new dates, with the same city pairs. Once again, Kayak had the lowest fare with the most choices and depth of information. Sidestep and Farechase had the same prices, with less info. Pricegrabber was a few dollars more expensive. Bezurk found nothing at all for North American itineraries. And Qixo? $120 more than the others. When the site even worked. Qixo came in last.