W3C's Web Accessibility Initiative (WAI)
http://www.w3.org/WAI/

• WAI Resources on Introducing Web Accessibility
• Quick Tips to Make Accessible Web Sites

The Details

• Web Content Accessibility Guidelines 2.0 (WCAG 2.0), in progress
• Web Content Acccessibility Guidelines 1.0 (WCAG 1.0)
  http://www.w3.org/TR/WAI-WEBCONTENT
• Techniques for Web Content Accessibility 1.0
  http://www.w3.org/TR/WAI-WEBCONTENT-TECHS
• List of Checkpoints for Web Content Accessibillity Guidelines 1.0
  http://www.w3.org/TR/WAI-WEBCONTENT-19990505/checkpoint-list
• Checklist of Checkpoints for Web Content Accessibility Guidelines 1.0
  http://www.w3.org/TR/WAI-WEBCONTENT-19990505/full-checklist