Wired Magazine recently had a good piece on people who game the airfare system to build frequent flyer miles. While you may not be interested in doing that,there are some good links in the article to help you find cheap flights. A few are below:
ITA trip planner – You can’t actually buy tickets from this site, but you can get really good information on cheap flights. ITA was developed by some smart guys at MIT who developed an algorithm that is apparently much better at tackling the super complexity of airline ticket pricing. It is the engine behind Orbitz. (Definitely try the one week or 30 day planner.)
Fare Compare – Great little tool for comparing fares of an itinerary.
FBOWeb – Get real-time flight tracking information from FBOweb. The plug-in for Google Earth is pretty cool if you want an air traffic control type of view of flight traffic near your airport.