A site describing itself as “A showcase and awards site featuring accessible, usable, universal, standards-compliant, and stunningly designed web sites.” Many useful links can be found on their Resources page.
The BBC’s guide to Web accessibility features and assistive technologies for users. Includes links and user reports from people with accessibility problems.
A description of the way people with disabilities actually use the Web. Very helpful in concentrating the mind on why accessibility matters.
The home of the W3C accessibility organization, from which you can find the relevant guidelines.
Adobe’s resources for making
PDF and Flash (more) accessible.