This site once won an award from a perl web hackers site – now defunct – for the 404 system.
The system was really simple. If you hit a 404 on this site it would apologise, then redirect you to a google search of Aquarionics for whatever you were looking for. This has worked for a while now, with the apologies getting ever more extravagant and the redirections being turned into google links (Because I hate being redirected places without a chance to stop it).
The latest iteration of this has just been finished (Yay the lunch break). It uses the Google API to display the first ten results alongside the apology. Finally a real use for the thing 🙂