Well, personally I would write it myself so I am not always the biggest help on finding premade scripts.
I don't think it would be an overly difficult thing to write. Once they register you have a cookie/session system to keep track of who they are. You could store their info in a db and then do what you like with it from there.
A custom system for this type of thing always makes more sense to me because the time spent looking around and testing and then customizing the script to your site will probably be more than if you wrote it yourself. One you create will also do everything you need it to and how you need it to.