You're probably referring to the New York Times slidebox. They were one of the first, or at least highest-profile sites to use the technique. There's a <a href="http://xaviershay.github.com/nytimes-slider/">demo here</a> of a plugin someone wrote, with link to Github for source. And here's a <a href="http://tympanus.net/codrops/2010/04/13/end-of-page-slide-out-box/">tutorial</a> implementing a simple version with just some custom JS.