HTML validators check your HTML against known standards. These validators do more than that - they check things such as broken links, spelling and grammar, page load times and much more. Some of these are online and some are off - but they all offer powerful tools for your Web site.
Doctor HTML offers many features for validating your Web pages, including: the complete structure, link checking, meta tags, cookies, spelling, images and more. The online version can even easily validate password protected Web sites. Doctor HTML is also available for license on your corporate Intranet. Thus you can easily validate your entire site before you post the pages to your live server.
CSE now has an online validator for checking HTML, XHTML, and CSS. And the offline version for Windows offers many features. Check multiple files at the same time and for the same errors. Your page is validated to indicate how it might look on a specific browser. Bad style is noted and with suggestions for improvement. This program is available as trialware, so you can try it before you buy it.
NetMechanic takes your site through all the steps you need to create a valuable resource for your customers. First you check the HTML and optimize your images. Then you optimize your page for search engine ranking and submit your page or your whole site to various search engines. When you're complete, you can purchase a site monitoring package to keep your site in shape.
HTML Power Tools is a complete set of tools for Windows to evaluate and manage your Web site. There is an HTML validator, link checker, and much more. One of the things I like the best is the multi-page search and replace tool. This is invaluable for making the same change across an entire site quickly and easily.
Dr. Watson checks any Web page that is publicly available online. It offers many options beyond just validating HTML. You can check your spelling (English), estimate the download times, check link popularity, verify links, find out how your site would do in search engines and so on. The prime drawback is that there is no indication of what version of HTML your site is being validated against.