Here's a great website for finding cheap domestic fares within the USA and Canada:
Orbitz: Cheap Travel, Flights, Hotels, Vacations, Car Rentals, Cruises
Be sure to tick the "nearby airports" check box. This site produced amazing savings when it substituted Buffalo airport (BUF) for Toronto (90 minutes away) for flights to and from San Francisco, New York and Vancouver.
It even tells you how many seats are left on each fare it finds.