SignUp forms that slide in

I would like to try to see how they work for us. I don't know if they are javascript or what. Here's some examples:
http://www.searchenginecollege.com/index.shtml
homeschool.com's internal pages (i think after they slide in once, they set a cookie so you don't see it again)
http://www.surfnetkids.com/

note, some of them are not up all the time.
Many are programmed to only slide in the first time you visit the site (in 30 days).