Considerate Not Found Page
A 404 page is an opportunity for a web designer. You have a user looking for specific content, and they haven't managed to find it.
Too many sites have a cute picture, or a pun, but don't help the user find what they want.
Have they misspelled a link? If you have a URL
/color and the user accidentally tries
/color), then you can help them!
It's a great opportunity to leverage search too. You have some search terms -- the intended URL -- are there any pages that contain that text?