Links for Accessibility Issues
W3C Web Accessibility Initiative
http://www.w3.org/WAI/

Different Disabilities that Can Affect Web Accessibility
http://www.w3.org/WAI/EO/Drafts/PWD-Use-Web/#diff

Dive Into Accessibility
30 days to a more accessible web site
http://diveintoaccessibility.org/

W3C Markup Validation Service
http://validator.w3.org/