Compliance tools help us to test our code against continually improving standards.
W3C HTML/XHTML Validator - basic evaluation of your website by the W3C committee
Style Sheet Validator - validate your CSS to ensure cross-browser validity
W3C HTML Validator - A free tool for testing your compliance with the HTML specification. Creating standards-compliant web pages is a significant part of accessibility.
W3C CSS Validator - A free tool for testing your compliance with the Cascading Style Sheets (CSS) specification.
Web Accessibility Management Tool (WAMT) - This is a free tool from the University of Illinois. It's still in beta, but is already one of the best web accessibility evaluation tools available. Unlike other free web-based tools, it can spider a site and provide evaluations of several pages
WAVE - Provides accessibility feedback with a visual interface
Web Accessibility Toolbar - This free tool operates as a plug-in to Internet Explorer (Windows only).
Color Blindness Simulation Tool - helps us see what pages look like to those with color blindness
Color Contrast Checking Tool - an objective test for whether your foreground and background provide adequate contrast
Firefox Web Developer Toolbar - adds a menu and a toolbar with various web developer tools
MAGpie - Free software for captioning multimedia