Launching this blog I needed to re design the 404 Error page.
I think an error page should give some information to the user.
In WordPress you can find a file called 404.php in your theme folder.
Now you can edit whatever you want but whatever is gonna change do it usefull.
In my case I use this code to show the latest 5 news posts:
See it in action: http://php.quicoto.com/12345