If you fly Southwest Air you can for a limited time sign up and get 4 credits. For those who don't know 16 credits gets you a free flight so getting 4 free is the same as getting 25% of a flight. I know it's not much but it will get you closer to a free flight on Southwest 25% faster than if you don't sign up.
I figure I have miles with 12 airlines now in "loyalty" programs so I might as well add another. Besides if other promotions come out I may be able to work closer to a free flight without even spending a dime.