The W3C's main production are its Specifications, but in the core of its philosophy is the belief that the natural complement to specifications is running code.
This page lists some of the tools provided by the W3C in the domain of Quality Assurance: validators and other test tools. Almost all of these tools are web-based, are available both as downloadable source, and as free services on the w3.org site.
W3C Markup Validation Service (HTML/XHTML)
http://validator.w3.org/
W3C CSS Validation Service
http://jigsaw.w3.org/css-validator/
W3C Link Checker
http://validator.w3.org/checklink
Feed Validation Service
http://validator.w3.org/feed/
more W3C QA tools are available at http://www.w3.org/QA/Tools/