These are my personal bookmarks of 404 resources. Most are IIS or inspiration sites. Basically it covers anything related to 404, that I havn't written an article on.
Making your website usable by your users should be the goal. 404 pages are part of the solution.
I don't use IIS (EVER!) but if you do here is a good resource.
Duncan Morris talks about the things developers need to remember before they launch a site.