Disability Discrimination Act – The Disability Rights Commission provides useful information on the legal aspects of web accessibility.
Outlaw.com – contains the popular 'Disabled access to websites under UK law' guide, and news articles relating to accessiblity and the law.
The Irish National Disability Authority IT Accessibility Guidelines - The Irish National Disability Authority website provides a plain English interpretation of the W3C guidelines together with the reasons behind why each checkpoint is important.
The Web Standards Project – The Web Standards Project website provides advice and information on the implementation of web standards and runs a campaign to encourage browser developers to ensure they adhere to W3C standards for code compliance. For an introduction to the rational behind standards they have an excellent educational FAQ at http://archive.webstandards.org/edu_faq.html.
Usable Web – The Usable Web website includes a collection of links about information architecture, human factors, user interface issues, and usable design specific to the World Wide Web.
Useit.com – Useit.com provides advice on both accessibility and usability featuring articles and news links.
Web Accessibility Initiative – The W3Cs Web Accessibility Initiative is a good place to start when learning about web accessibility. It offers an excellent resources section, together with the Web Content Accessibility Guidelines and Checklists.
http://www.w3.org/TR/WCAG10/ – Web Content Accessibility Guidelines.
http://www.w3.org/TR/WCAG10/full-checklist.html – Web Content Accessibility Guidelines Checklist
http://www.w3.org/WAI/Resources – The resources section is a good place to start learning.
WebABLE – The WebABLE website offers a wealth of disability-related internet resources, including an extensive (North-American biased) database on all aspects of accessibility.
WebAim – WebAim, the Web Accessibility in Mind Project provides a number of articles, a discussion forum and a number of simulations which is useful for testing websites.
The Web Design Group are excellent at demystifying the W3C recommendations and providing tutorials on accessible implementations.
For web developers
A List Apart – resources for people who make websites.
It also hosts the Accessify Forum which is an accessibility related discussion group.
AskTog – AskTOG by Bruce Tognazzini is perhaps the best place to start learning about human computer interaction (HCI).
Aware Centre – (Accessible Web Authoring Resources and Education) serves as a central resource for web authors for learning about web accessibility.
Dive Into Accessibility – Dive Into Accessibility is an online book which discusses accessible design issues and techniques.
Juicy Studio – Juicy studio provides a repository of useful articles on accessible techniques and practices together with some useful tools to assist developers in creating accessible content.
W3C Schools – Whilst not specifically about web accessibility this website includes many tutorials on learning HTML, CSS, XML and much more.
Webvisum provides community tagging and a Captcha solver.