HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
Dr. Link Check - Broken link checker with malware and parked domain detection
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
BrokenLinkCheck - Free Broken Link Checker / Online Dead Link Checking Tool - find bad hyperlinks
Rocket Validator - Site-wide Accessibility and HTML validation made easy. With a single click, Rocket Validator will check thousands of web pages on your site for Accessibility and HTML conformance, producing a detailed, actionable report on seconds.
LinkChecker - LinkChecker is a free, GPL licensed website validator.