Web Site Garage
From tune-ups to turbo-charges, the Web Site Garage has everything to keep your site running smoothly. Use the link and spell checkers, HTML validators, and the GIF Lube, which reduces the size of your images so your pages load faster. You can also get help registering your site with search engines, publicizing your site, and get automatically generated META tags. A Tune-up, a limited spellchecker/HTML validator, is free. If your site requires more intense work, Web Site Garage can do that work for a fee.
This site's easy-to-use robot will check the links on a single page or a whole Web site. NetMechanic also has a separate page for HTML validation
Like the NetMechanic, Doctor HTML fixes ailing HTML code and validates links; it even checks spelling and syntax for you.
Builder.com — Put Your HTML to the Test
Builder.com's done it again! This comprehensive, well-researched article has reviews of seven different checkers, and instructions for how to use them.
The WC3 Validation Service
Use this site to check your HTML for compliance with HTML 4.0 standards.
Watson snoops around your site and verifies your links and investigates your spelling. It's elementary!