Update December 9, 2007:
The freetraveling.com site is dead. So the list of promotional codes for American Airlines and other carriers, linked below, is down. However, there IS a 5% discount codes for American Airlines, as well as a discount for US Airways Vacations, in the 2008 edition of the Entertainment book, which may be a worthwhile investment.

The text of the original post follows, as originally published.

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The folks at FreeTraveling.com have compiled a list of promo codes (Edit: link is deleted, as the site is dead) for several U.S. airlines.

Most of the discounts are for American Airlines, but I found codes for Continental and Northwest as well.

To find your code, you need to select your destination city from a pulldown menu first. At this point, there are about 40 cities with promo codes.

Discounts aren’t always available for every time frame, much less on every airline, but it’s worth taking a look before you buy.

To use the codes, you may need to select “advanced” fare search options on the airlines’ websites. They don’t always make it immediately apparent.

Alternate way to get a discount: Buy an Entertainment book. The price of the book varies according to the “home” location, but each book contains a 5% discount code for American Airlines.