HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
Xenu's Link Sleuth - Xenus Link Sleuth checks Web sites for broken links.
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Scrutiny - Scrutiny for Mac helps you manage and improve any number of websites. Link check, search, full website spell check, XML sitemap generator, SEO audit
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.