Web Accessibility Initiative
http://www.w3.org/WAI/
The Web Accessibility Initiative (WAI), in coordination with organizations around the world, is pursuing accessibility of the Web through five primary areas of work: technology, guidelines, tools, education & outreach, and research & development. Includes link to Techniques for Web Content Accessibility Guidelines (9/20/00)
Trace Center: Designing more useable web sites
http://www.trace.wisc.edu/world/web/index.html
Macromedia Accessibility Kit
http://www.macromedia.com/macromedia/accessibility/tools/accessibility_kit.html
Download the Free Macromedia Accessibility Starter Kit, which includes guidelines on how to create an accessible Web site or retrofit your existing site to comply with Section 508 requirements.