A 404 error page is that annoying page you get when you try to access a web page that has moved or been deleted.  A couple of months back I wrote a dorky blog post about how web developers should use  404 pages as branding opportunities instead of just throwing the page away and spitting out an error.  You can read the post here if you are interested.

We recently launched a new website for the Orianne Society, and our Bivings design team put together another stellar 404 page (screengrab below).

Update (3/7/2011): Here is another example from another TBG client, the Entomological Society of America.

Smashing Magazine has other great examples.