Something useful to do with 404 pages
Clever: Why Not Use Your 404 Pages To Help Find Missing Children?
Here is an example.
Notfound.org/notfound
They are hoping for a link back to notfound.org, and you get a link from them as a participating site, at Notfound.org/participants
I checked a couple unlikely urls on the page and it seems they will accept almost anyone for the link back.
> DON'T CLICK THIS LINK <
Don't worry be happy!