Lorelle writes about the W3C Multipage Validator. Validating markup of the entire site with the traditional validator was time consuming and usually led to skipping some pages. The multipage validator makes it very convenient. It internally uses the same validator, but indexes multiple pages of your website and reports using AJAX. You can give your feedback and contribute to the tool. A great find, thanks Lorelle!
Some additional tools that you can use for your website are:
And there is a big list of accessibility tools with W3C. Feel free to add more to the list.