Pixelpoke shares a whimsical Page Not Found link. While it's definitely grin-worthy, and therefore a wonderful way to begin the work week, it still can't displace my favorite customized 404 error message -- the simplicity of it tickles me. (And it was really really funny ten years ago...)
GameWyrd has some amusing suggestions for the 404 error. I liked EffieRover's "This is the refrigerator in www.gamewyrd.com's coffee room. The web server is down today, so I'm standing in for it. I couldn't find your file on any of my shelves, but I'll stick a note to myself with one of these magnet thingees to notify the webmaster."