Google's custom 404 widget
I've just found out that Google has released a custom 404 widget. The idea is to improve the experience of 404 pages and instead of telling the user that he or she is lost, provide him with something he could really use.
Google's custom 404 widget resembles other Google technology like search. All you need to do to integrate it in your website is to copy & paste an HTML snippet. Check out the Google custom 404 widget at Google Webmaster Tools.
Unfortunately it's impossible to provide a direct link to the Google custom 404 widget so after you'll go to the Google Webmaster Tools page you have to choose Tools > Enhance 404 pages from the menu. Talking about useful tools...