They're relatively smaller and not as well known, but Sun Country Airlines promises affordable flight prices. They also promise exorbitant baggage fees and the development of a weird kink in your neck. The seats lean forward during the flight, and unless you're a youthful 19-year-old with no back problems, this is not the airline for you.
Editor's note: The seat issue aside, Sun Country really is affordable if you're flying around the Midwest. Minneapolis-St. Paul is their hub, so flights to and from MSP are a bargain.