There are a number of low cost airlines. It depends on where you are flying out of and where you are traveling to. Southwest is the biggest low fare airline. It flies out of and to the most destinations. However, it often flies out of smaller airports in some cities. For example, it doesn't fly out of any of the big 3 airports in the NY area, instead it flies out of Islap airport on Long Island.
There are other low cost airlines like Air Tran, ATA, etc. If you tell us the airports you'll be flying out of and into we can probably give you more assistance.