W3Clove project: http://w3clove.com
A free online service that lets you validate the HTML markup of your whole website with just one click.
The system it uses for scraping sites, validate its pages and produce summary reports of the errors and warnings found is available as an open source Ruby gem
I need to collect money to pay the hosting fees so I can keep this service free for everyone. Right now I’m paying it all by myself but this is not sustainable in the long term, and I don’t want to place ads on the site. Here is an expense report that I update monthly: