Web Site Compliance with the Americans with Disabilities Act (ADA)

I'm in agreement with @Jim that accessibility compliance is at the moment is not a 100% objective science.

Take the classic case of image alt text. Suppose a story about education in America includes a closeup photo of a smiling female Hispanic student, approximately ten years old, at a desk in a classroom writing on a piece of paper with a pencil. The WCAG1 guideline says to include a "text equivalent" for every image. Some would suggest that alt="young Hispanic girl at a desk in a classroom" would not provide enough meaning to convey equally what the photo shows. Others would argue that it conveys too much, that it's just a stock art filler and that alt="girl at desk" is appropriate. Others would argue that the photo represents a point made in the article and that alt="Education policies of the past 10 years have affected minority students most." is the most appropriate alt text.

So even experts in the field can disagree on what is best for this one simple situation. Automated tools can only detect whether alt text is missing -- not whether it provides a text equivalent. And we can all agree that placing alt="[image]" does not meet the intent nor the guideline of providing a text equivalent.

There are some online checkers that can test for some of these issues. Chris Pederick's Web Developer Extension for Firefox provides easy access to test against WAI and 508 -- as does the IE Developer Toolbar.

No automated tool can tell you whether a website is accessible. There are tools, such as Rational Policy Tester, that can identify potential problem areas, but they only work in conjunction with manual checking by a person with a good understanding of the requirements. A good place to start looking for your tools is at the WAI.