Accessibility testing is a form of usability testing to verify your website is usable by all people. Check out the W3C's Accessibility Testing page for information on when testing should be done, testing requirements, who should test your site, etc.
Use the following tools and testing producers to evaluate your website for accessibility:
Accessibility Evaluation Resources is a multi-page resource by the W3C that outlines different approaches for evaluating websites for accessibility.
Test results showing how different content types and standards behave in assistive technologies (AT) like screen readers.
A series of quick checks by the WAI that help you asses the accessibility of a web page.
A quick reference to Web Content Accessibility Guidelines (WCAG) 2.0 requirements (success criteria) and techniques.
This site tests which new HTML5 features are accessibly supported by major browsers.
Use webhint to check for out-of-date libraries, security improvements, performance problems, accessibility issues and more.
A Visual Studio extension that performs accessibility checks on any ASP.NET web application.
A list of web accessibility evaluation tools to help determine if websites meet accessibility guidelines.