From Richard Miller's blog today, there's a new post with content from the Utah PHP Users Group meeting that's just passed (Feb. 2006) on the topic of custom 404 pages.
When you try to visit a web page that doesn't exist, the server usually returns a 404 error message - "Page Not Found". At the Utah PHP Users group meeting last Thursday, Mac Newbold presented on custom 404 pages. Custom 404 pages can help maintain your website branding, help you fix broken links, and help your users find something useful when they don't find what they're looking for.
Custom 404 pages are even more powerful when you combine them with PHP. I certainly didn't expect the flood of ideas that Mac presented for using custom 404 pages.
Among the suggestions of things to do with the error pages are things like: sending an email when someone gets a 404, search the site for something similar, use it as a redirect for old to new URLs, and more.