from a friend....
cheapfares.com is good and I use that and Kayak.com pretty much if I am traveling within and to the US. Traveling to Europe I also try those but supplement it with BT-Store.com - great prices if you plan ahead.
I also ALWAYS check the small independents that are typically not listed on those sites, for instance vivaaerobus.com or volaris.com.mx for travels in Mexico or Spirit, Jetblue, etc. in the USA. Some savings are incredible. Recently Viva Aerobus had a sale between Cancun and Mexico City for less than US$20+tax roundtrip. It was a penny plus tax one way - incredible.