Check out southwest.com (assuming they fly to the airports you need). Install their Ding! application off their website to get even better deals. Not only are they usually the cheapest (I think they're having a sale right now too) but they don't charge for checked baggage like nearly every other airline.
For my trip next week from SLC to AZ to San Fran and back to SLC, the cheapest I found anywhere else online was about $330 with taxes and what not, plus it was Delta so I would have paid like $15 bucks per checked bag. I waited and kept an eye on Southwest Ding! and got all 3 legs for a grand total of $250 without any checked bag fees. Whenever I fly somewhere, I spend hours looking for the cheapest flight, but it has always been Southwest.
