Research-Based Web Design & Usability Guidelines (PDF) – A step by step guide of principles for developing usable and useful web sites. Serves as a great checklist for reviewing your designs. Online version also handy: http://www.usability.gov/
The Usability Methods Toolbox – A compilation of the techniques and methods of usability evaluation.
Guidelines for Accessible and Usable Web Sites (PDF) – A compilation of research-based guidelines for accessibility and usability in web site design.
User Experience Design Process – A PDF outlining the flow of the UX design process.
Web Style Guide – I hate the site design (ironic, eh?) but the content is a simple primer on the basic principles of web site design.
The Usability Toolkit – A collection of forms, checklists and other useful documents for conducting usability tests and user interviews.
Gerry Gaffney’s Information and Design – A variety of usability resources and templates.
Interactive Usability Methods Table – An interactive table of usability methods that gives you the best choices for usability testing based on your constraints.
User Experience Diagrams – Luke Wroblewski’s exhaustive list of User Experience Diagrams.