I'd like to add TotalValidator (free cross-platform app) and Fangs (FireFox extension) to the list.
I would also like to emphasize that because accessibility is tightly tied to usability and no automated WCAG validator will replace a human trying it and breaking it. We're continually surprised by some of the problems that have appeared during usability testing even when the code is WCAG compliant.
The flip-side to that is that you need to interpret the results from usability testing, as people always express their personal preferences (which may have no real impact on the design's general usability or accessibility compliance). In this field, you may come across more ... passion than in other usability testing.