The Web Accessibility Initiative defines guidelines for making websites accessible to people with disabilities. See an overview explaining the importance of web accessibility and the WAI guidelines. Probably the most well-known validator is Bobby, which is available as a web-based tool for a page at a time, or you may purchase a copy and run it on a whole site in one chunk. I downloadable tool that I like is Wave, which allows you to add a button to your browser to instantly check the page you are viewing, it gives a handsome graphical analysis of your page, and it let's you continue browsing your site through its analyzed page! Drag this button -- Check Acc -- onto your IE toolbar to give one-click validation. (This page provides instructions for installing in other browsers.) Next, A-Prompt is a good downloadable package for Windows which provides the most configurable environment, allowing you to specify level 1, 2, or 3 validation, and even has an automatic repair capability. The Infocus package from SSB Technologies is perhaps the Cadillac of accessibility checkers; they not only sell an off-the-shelf product, but provide custom solutions to incorporate any additional guidelines and guidelines required by your company, if you work in the corporate world. Finally, using the standard tool for text-only browsing -- Lynx -- is always a good way to check that your web page is accessible. You can even add a button to your browser toolbar to run your current page through Lynx -- Check Lynx
Validating for accessibility, unlike validating HTML and CSS, is difficult to automate. Some items may be checked programmatically, but some cannot really be. The better packages give you a good list of things to examine, though. Once validated, you may add the W3C seal of approval to your page, an image indicating that your webpage conforms to web accessibility standards. There are three levels of compliance: the W3C wishes to encourage compliance without making it an undue burden, as meeting the most stringent requirements could cause an inordinate burden on some sites and some companies. See the guide to Conformance Logos for more details.