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404
A site's 404 page is one of the best places to express creativity and keep the user entertained, and you don't have to worry about consistency and such, because nobody is supposed to see it right?
Anyway here is a collection of the most widely used examples:  http://blogof.francescomugnai.com/2008/08/the-100-most-funny-and-unusual-404-error-pages/

And if you want some more words around your examples to make one too, take a look here:  http://www.smashingmagazine.com/2007/07/25/wanted-your-404-error-pages/ and here  http://www.smashingmagazine.com/2007/08/17/404-error-pages-reloaded/

But the reason I wrote this post is because of the coolest one of them all, the one from Github:  https://github.com/404.
And because they might change it and just for fun here it is embedded in the blog post:

Btw. Did I mention you need to move your mouse over it?