W3C has updated its W3C Markup Validation Service and Link Checker with bug fixes, documentation and usability improvements, and a new Validator API for developers. Along with W3C's other Web Quality Tools, the Markup Validator and Link Checker are developed as open source software with the participation of volunteers and support of a large community, and are among W3C's most popular and useful resources.
Links:
Validation service: http://validator.w3.org/
Link checker: http://validator.w3.org/checklink
W3C Web Quality Tools: http://www.w3.org/QA/Tools/
