I came across a creative 404 (page not found) error page on Digg this morning. It made me wonder what other sites have creative 404s. If you come across this post, feel free to pop an entry in the comments and I'll add it to the list.
Here's a start:
http://homestarrunner.com/systemisdown.html (not their actual 404, but creative nonetheless)
http://homestarrunner.com/404 (their actual 404)