Notfound.org
This wouldn’t be appropriate for every site but I still think it could be a damned fine use of otherwise-neglected 404 pages: including information about missing children.
Link tags: 404
3
CS404
This looks like an excellent event: learn about programming without being a programmer.
sneeu.com // Fuck.
I know it's childish but I think this may be my favourite 404 page ever.