The guys from watchfire.com are running a service called Bobby that does an automated accessibility check. A robot that checks web sites for usability? My first thought on this was something like "Nice toy, but...". Now that I had a look on the thing I must admit that the results are impressive.
There are several different check levels available. This page passes the generic A test and the 508 test. The latter checks for conformance with the U.S. Section 508 Guidelines. The site is commercial, which means they want to sell the desktop version of their validator, but the online check is free for one page per minute.
This seems to be a nice addittion to the usual w3c conformance tests and other syntax validators that I feed with my web sites from time to time to make sure there are no slips in my markup. And then, they give me just a few more buttons that I don't know where to put.---