Creating useful 404 (Page Not Found) pages

Madhu Menon posts a useful article on ways in which to best guide users when they encounter a webpage that doesn't exist or has been moved from a website. The article shows examples of bad use and also recommends techniques on what best features to adopt. The 404 Research Lab is another useful resource site on Best Practices with guiding users when errors occur, it also has a Pros, Cons sections and a History of the 404.