Sign up for a new Frontier Airlines EarlyReturns account here and earn 1,000 free miles by enrolling and completing a form with information about yourself. When signing up for your new account, click on the box for Frontier Sales & Specials and you will be notified of their deals by e-mail.
Frontier is a points transfer partner at Points.com, meaning you can move points from Frontier into other loyalty programs (at a substantial devaluation).
Sadly they have since imposed a minimum threshold where your account needs at least 1001 miles in it in order to be able to transfer points out!
Update: I wasn’t going to state the obvious, but since Anon256 spelled it out in the comments…
The 1001-mile requirement isn’t that much of an issue, just transfer 5 or so miles from AA or US to 1 Frontier mile, now you can transfer the 1001 miles back out to 150ish miles elsewhere.