Unicorn – Universal Website Validator is a nice new validator tool from the W3C that runs their suite of tests on your site, checking for accessibility and standards compliance.

Try it out for this site! – lots of CSS errors due to my use of vendor specific prefixes, which I personally think, used with caution, should be let through.

Also try out the Mobile Conformance option for an overview of how well your site conforms to mobile device best practices.