Kayak.com is what I usually use!
BUT, this is what I have to say...If you want the cheapest flight you may have to drive some. Like us...For the last 4 years when weve flownto Ohio and Worlds, weve never flown out of our Home Airport (DFW), because another major Airport had a WAY cheaper flight.
Weve normally had to drive 4+ hours and saved roughly $200-300 a flight on the cheaper flights, and when we went to Hawaii, we saved nearly $600 a ticket!!
ALso, be sure to cnstantly check the Main Airlin ewebsites, you never know when they may have a low day...