ITA Software's Matrix is the software that powers Orbitz, Kayak, CheapTickets and lots of airlines and travel agents' tools. But you can skip all their ads and come-ons and search Matrix itself for the no-nonsense scoop on flying cheap.
There's not much to not like about how Matrix gets its job done. You know how you're asked to "Please wait while SuperDuperFlights finds you the best fares!" That wait is cut nearly in half on Matrix. You can view a standard comparison of airline fares from one city to another on certain days, but if you're looking for something a bit more specific — flying in and out of multiple cities, for instance — the "More Options" button lets you do what you will across the airways.
The calendar view of the cheapest days for flying is another handy feature, though that's offered on other flight comparison sites, too. Commenters at The Consumerist blog point out that hardcore business travellers can get a lot more out of Matrix by learning some of its simple flight codes.
How about it, frequent flyers? Are you Matrix masters, or do you use another tool to get around the world (personal assistants don't count). Matrix is a free site, with an iPhone app offered, too.