AirTran has one of the newest fleets in the sky... mostly 717's (formerly the MD-80 series before McDonnell Douglas was purchased by Boeing) although they have purchased some 737-800's for their longer routes. Unfortunately, you won't find any more legroom there than on the major airlines. Except for a few routes to the west coast, this is primarily a short-haul airline.
Although they're headquartered in Orlando, their primary hub is here in Atlanta. We love them. They're forcing Delta to be competitive!