On the Web Development Blog today there's a new post about 404 pages and some of the things you can do to make them a bit more useful than just a "not found" message.
After your websites getting more pages and links, the chance that a visitor will follow a dead link to your site exists. If a visitor is trying to access a page on your site, the server will report (normally) a 404 error. The response is by default some unfriendly page with some spare information about the error which let most visitors stop visiting your site. But using the 404 error the right way, you the site owner can collect important information.
He talks about some of the data that could be collected - what page they were requesting, where they came from, access statistics and more. The tutorial shows you how to harvest some of this information and how to put Google's Search and Adsense tools on the page as well as Google Analytics for tracking.