I have a weakness for hexagons: https://floobits.com/404

The 403 and 500 pages are a little more aggressive: https://floobits.com/static/403.html https://floobits.com/static/500.html

I think I've picked a nice combination of utility and aesthetics. The game on masswerk.at's 404 page is pretty cool, but I could see it being a little distracting or confusing for some users.