The official reference: http://www.section508.gov/ Section508 of the Rehabilitation Act of 1974 (as amended, 1998). Includes the “Buy Accessible” reference section for procurement officials, product vendors and service providers.

The Access Board http://www.access-board.gov/ is an independent federal agency devoted to accessibility for people with disabilities. It operates with about 30 staff members and a governing board of representatives from Federal departments and public members appointed by the President. In particular, take a look at their “Electronic and Information Technology Accessibility Standards” page at http://www.access-board.gov/sec508/standards.htm

WebAIM (Web Accessibility In Mind) http://www.webaim.org/standards/508/checklist has a concise checklist of accessibility features required for websites under Section 508 and Pass/Fail criteria.