The only others I can add that haven't been mentioned are
www.flyi.com www.flysong.com (Delta affiliated)
Another tip: go to
www.expedia.com or
www.travelocity.com or
www.orbitz.com and type in where you want to go. THEN go to that airline's website of the cheapest one. When you book directly from that specific airline's site you save at least $5 per ticket sometimes more. But remember those comparison sites don't bring up the economical airlines like southwest, jetblue, etc. So you really just have to check around, then make a choice. LMK by pm if I can help you. I'm pretty good at cheap flights and transportation travel.