My friend did one. Had to get 5 people to sign up for some different free trials (stamp collecting, blockbuster video, etc.)
At the end he got a free 20GB Ipod.
So, while most probably are a crock, some of them are legit. I read an article about it a while ago, it was specifically on the Ipod deal my friend got. I guess the company, although they have to pay for the Ipod, still ends up with a killing.