Writeboard
37signals released yet another web application today. For an overview I’d recommend their blog post and the tour. The “oh-that’s-cool-and-could-be-useful-in-lots-places” part of Writeboard would have to be the different size circles beside each revision. On the list of revisions they show a circle, which depending on its size tells the user how drastic of a change each revision was. See #4 of the tour.