Unix, Windows, MacOS, and others. Validator fixes errors in HTML and XHTML. Converts HTML to XHTML. Free Software.

Windows. HTML, XHTML, and CSS validator. Also checks accessibility, links, and spelling. Commercial pro version and free lite version.

Windows and Unix. Object-oriented toolkit for SGML parsing and entity management. Can validate any version of HTML for which a DTD is available. Free Software.

A W3C Tidy-based HTML Validator which checks pages from within the Firefox web browser.

Unix and Windows. Lint and link checker. Commercial software; free trial. Also online checking.

Windows. Includes validator, image scanner, meta tag manager, find and replace tool, tag pair fixer, and an HTML to text converter. Commercial software; 30-day demo.

Perl script which corrects incompatible HTML generated by Microsoft applications.

Linux, Mac OS X, and Windows. Finds broken links, non-standard HTML, and links between URIs on web sites. Commercial software; free for small sites.

Windows. Optimizes HTML documents removing unnecessary HTML code while checking links. Commercial software with free trial.

Windows. Instructions for the Do-it-yourself Offline HTML Validator using SP.