(MONEY Magazine) – All frequent-flier programs are not created equal. While most airlines impose ''capacity controls'' -- limits on the free seats awarded to frequent fliers -- several have none. Midway FlyersFirst has no capacity controls (on Midway flights only). MGM Grand Air, the luxury carrier for bicoastal commuting, is the same. At least four other programs -- Alaska Mileage Plan, America West FlightFund, American AAdvantage and United Mileage Plus -- offer special awards in exchange for higher mileage.