CSE HTML Validator Professional
CSE HTML Validator Professional is a fast, powerful, highly user configurable, and easy to use professional web development tool that helps you create syntactically correct and accessible HTML, XHTML, and CSS documents. The new HTML Tidy Tool can even fix HTML and XHTML (and more). ... The latest version of CSE HTML Validator includes accessibility checking (Section 508 and WCAG), CSS syntax checking, link checking, spell checking, a powerful, easy to use, integrated HTML/text editor, an integrated web browser (browse the web while you validate the pages), and a powerful Batch Wizard that...
ftp://ftp.htmlvalidator.com/pub/htmlval/csesetup.exe - 4Mb - Download - Screenshot

Total Validator Tool
Why use separate validation tools to check your web pages when you only need one? The Total Validator Tool performs the most important validations on your web pages: True (X)HTML validation against published W3C/ISO DTDs; accessibility validation against the W3C and US Section 508 guidelines;... You should ensure your sites conform to the W3C WAI Accessibility Guidelines, (and that also means strictly W3C validated HTML), or US Section 508 standard, if you want your site to be accessible to all.
http://www.totalvalidator.com/downloads/TotalValidator_F200.exe - 2Mb - Download - Screenshot