This is a nice diagram of the user centered design process and an overview of the methods and techniques which might be used. The HCI toolkit is managed by Bas Leurs for Rotterdam University of Applied Sciences.
Smashing Magazine expound upon what makes a well designed 404 page in their latest design gallery which features 50 examples.
Inspired by the gallery, I made a more useful 404 for Konigi as well.
http://www.smashingmagazine.com/2009/01/29/404-error-pages-one-more-time/
Fubiz has done a beautiful redesign. The gorgeous video below showcases the redesign, giving you a taste of what to expect as you begin to use it. I'm really digging the Mosaic view and the Popular carousel in the sidebar.
Fubiz Redesign from peresini mattias on Vimeo.