W3Clove is a cool web application for conducting a site-wide W3Schools (HTML/XHTML) validation.
The only downside of the website is that it can only check a single URL at a time, and seems to have all the time in the world to scrape data for some websites. The validation is done through the W3Schools validator.
The generated list is easy to use as you can click each error and view the pages the errors are in.
- Website: http://w3clove.com