Depending on your location, AirTran has been one of my favorite Low Fare Airlines. They fly MOSTLY BRAND NEW Boeing 717's, and are a fun airline to get from here to there. Also quite easy to earn free flights.
the best way yahoo travel ,it will give you all the airline and prices based in your distination
It depends on where specifically you are going. My personal favorite is Jet Blue, their prices are great and so is their service. They've even helped me out when I was stuck in San Diego (when American Airlines wouldn't honor my ticket, don't fly American).
I would definitely look into JetBlue.
Either Southwest or American airlines.
You might want to try Yahoo's farechase. I was just looking for a flight to Europe, and I found their program very easy to use and helpful.