Validation Tools

CSS

CSS Validation

XHTML

XHTML Validation

Accessibility

Note that none of these automated tools are a substitute for a comprehensive accessibility review. Many of the 508 and WCAG requirements are subjective and are difficult or impossible for a machine to assess. In the final analysis, only you and your visitors can determine if your site is truly accessible.

Bobby
Possibly the most popular automated accessibility checker.

Cynthia Says
Another automated accessibility checker.

ColorBlind Web Page Filter
Test how your pages look to people with various forms of color-blindness.

Juicy Studio Color Contrast Analyzer
Another tool for testing color and contrast.

Reference

Web Accessibility Testing by Jim Thatcher