I thought it might be useful to create a master list of frequent flyer/rewards programs that give you a bonus just for signing up for them. There aren't a lot, but there are definitely a few I've come across, and having a list might help those who haven't signed up yet get bonuses. So, here goes (perhaps it might be useful to make this a wiki?)

-Amtrak Guest Rewards - 500 points just for signing up, 500 points for the person who invited you if you are "invited"
-Fairmount Club - you can get 1000 Aeroplan points just for signing up
-I believe Marriott rewards had some kind of offering of a sign-up bonus if you were referred?