W3C has a new accessibility section. It does a nice job of unifying their accessibility content into one location. The home page has a great explanation of the three W's of accessibility. Why, What, and How. (Two W's and an H just doesn't have a nice ring to it.)

"The Web is fundamentally designed to work for all people, whatever their hardware, software, language, culture, location, or physical or mental ability. When the Web meets this goal, it is accessible to people with a diverse range of hearing, movement, sight, and cognitive ability." - Accessibility - W3C