Update:
The usairways.com discount below expired at the end of 2006. So the list of promotional codes for American Airlines and other carriers, linked below, is down. However, there IS a 5% discount codes for US Airways Vacations, as well as a discount for American Airlines, in the 2008 edition of the Entertainment book, which may be a worthwhile investment.

The original post lives on below for posterity.

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If flying US Airways, and booking on usairways.com, try entering the promotional code RR506FS to receive a 10% discount on purchased first class tickets, or promotional code RR506CU for 5% off economy class tickets. On the flight booking page, enter these codes in the “e-certificate” box located below the “return date” field.

Travel by November 15. Up to 4 passengers can receive the discount on the same itinerary. No codeshares. It doesn’t appear to have any advance purchase or geographic restrictions — you can seemingly use it for any destination on US Airways. I also don’t see any reason why the codes can’t be used again and again, so have at it.

Main offer page here [links deleted, offer expired]; full terms/conditions, and (important!) blackout dates here.