BetterHistory is a small Javascript extension for Wikipedia that allows you to smoothly view past article revisions. It's only a beta version, but I like it already. So there's that...
The current version is 0.6, released June 21, 2005. (changelog)
Update, 22-June-2009: The links below have been broken for quite a while, but I was able to find an old copy of the script today after a bit of searching. When I have a chance, I'll try to update it so it works properly again.