Whilst visiting Digg, I came across this site, basically it allows you to redirect people who have come onto your site via Google only to find a 404 error to a valid page (damn, I dont think even I can understand that last sentence!).

Its good if you, like me, have taken over a website from another coder and dont know what pages are actually in use (mine is a particularly large site) and want to keep users on your site and viewing valid content.

Anyways, take a look @ http://www.stevenhargrove.com/redirect-web-pages/