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Web Pages That Suck
A humourous but very informative site which aims to teach usability and good Web design by looking at bad design
useit.com: Jakob Nielsen's site (Usability and Web Design)
The self-proclaimed 'guru' of web usability. Often quoted and imitated.
IBM developerWorks : Usability
IBM have an excellent usability department, with a lot of interesting articles.
Usable Web
Usable Web is a collection of links about information architecture, human factors, user interface issues, and usable design specific to the World Wide Web.