W3C WAI Documentation and Guides
Web Content Accessibility Guidelines (WCAG) Overview
Web Accessibility Automated Checkers and Validators
As with all automated validators, these tools provide a beginning for a web accessibility review. They, however, cannot be relied on to provide a complete assessment of web page accessibility. The key to using these tools successfully is to understand how to interpret the results and then proceed with a finer grained analysis.
Web Accessibility Analytic Tools
Firefox Accessibility Evaluation Toolbar
Paciello Web Accessibility Toolbar (WAT) for IE