Uh yeah, that already exists:

http://www.stumbleupon.com/toolbar/ (random page)

This won't take off at all. If you still want to pursue this idea, least you could do is add a top iframe with some info on the site, comments, etc..

Edit: well, you said in your original comment that there would be a frame, but it doesn't seem to work on the current page that is displayed. (safari, mac)