I'm doing a web site where one of the requirements is Section 508 compliance and I wanted to find an online validator similar to W3C's XHTML and markup validator.

So, I found two online accessibility validators that seem to work well:

http://www.contentquality.com/Default.asp

and

http://bobby.watchfire.com/

Also, Stephen Walther has a good article on MSDN about how to configure your ASP.NET pages and controls for accessibility (credit Brian Russell with the link).