Validators
Validators check your code to make sure it is written correctly. If there are any errors, the validators indicate where and what each problem is. You should validate all your pages to make sure they are compliant with the Web standards.
HTML
CSS
Accessibility
- WAVE (provided by WebAIM)
http://www.wave.webaim.org/index.jsp
Test for free - WebXACT (provided by Watchfire)
http://webxact.watchfire.com/
Test a page at a time for free - Complete list of accessibility tools (WAI - Web Accessibility Initiative)
http://www.w3.org/WAI/ER/tools/complete