Accessify - A very good informative site about making your website accessible. Full of news articles and reports.

UseIt - A leading authority on Usability issues.

The Plain English Campaign - Working towards improving readability, they can offer help in making your documents readable.

A List Apart - Online magazine about web design and web standards compliance.

How People With Disabilities Use the Web - The World Wide Web Consortium offers examples of how people with different disabilities access the internet and problems they encounter.

WebAIM - News, information and tips about accessibility.