WebXact - Formerly Bobby, an automated web accessibility checker.
Wave - Web Accessibility Versatile Evaluator. An automated testing tool brought to you by WebAIM.
Cynthia Says - An automated accessibility testing tool.
Color Vision - Look at different colour combinations of backgrounds and text to find the right contrasts.
Vischeck - Test how your site looks to users with different types of colourblindness.
Plain Language Audit Tool - Assess how readable your site is.
Fangs - A Firefox/Mozilla extension which allows you to linearize your pages.
Accessibar for Mozilla/Firefox - Toolbar which allows you to manipulate the settings of any webpage in your browser.
Web Accessibility Toolbar for IE - A useful toolbar for checking various accessibility issues such as CSS, alt texts and colour contrasts by changing the settings on your browser.
Web Developer Toolbar - A toolbar for Firefox/Mozilla that lets you check your pages for missing alt text, turn CSS, images and scripts off and on, validate your pages, etc.