Fact: simple 404 pages look like they’re from the Altavista-era Internet. This leaves a scar on user’s experience with your website. Modern users want smart, witty and actually helpful ‘not found’ error pages.
Don’t be the guy who says ‘you broke something’ on his 404 page. User shouldn’t feel guilty about you or your machines not being able to find what he was looking for. More often than not, this is actually your fault – moving things around and not accounting for some pages linking to the content.
Say you’re sorry, explain why the user is here, add some links for a way out that does not involve clicking the red button with a cross in it and entertain your user in a way that will overpower any negative feeling they might have after learning that the page they’re looking for does not exist.
Without further ado – the best 404 pages on the interwebs.