Validators and Lints

CSE HTML Validator
Windows. HTML, XHTML, and CSS validator. Also checks accessibility, links, and spelling. Commercial pro version and free lite version.
http://www.htmlvalidator.com/

Demoroniser
Perl script which corrects incompatible HTML generated by Microsoft applications.
http://www.fourmilab.ch/webtools/demoroniser/

Firefox extension HTML Validator
A W3C Tidy-based HTML Validator which checks pages from within the Firefox web browser.
http://users.skynet.be/mgueury/mozilla/index.html

HTML Code Cleaner/Compressor
Windows. Optimizes HTML documents removing unnecessary HTML code while checking links. Commercial software with free trial.
http://www.htmlcodecleaner.com-http.com/

HTML PowerTools
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.
http://tali.com/

HTML Shrinker
Windows. Removes unnecessary characters from HTML, CSS, Perl, JavaScript and many other web file formats. Commercial pro version and free light version.
http://www.thepluginsite.com/products/htmlshrinker/

HTML Tidy
Unix, Windows, MacOS, and others. Validator fixes errors in HTML and XHTML. Converts HTML to XHTML. Free Software.
http://www.w3.org/People/Raggett/tidy/

LinkScan/QuickCheck
Unix and Windows. Lint and link checker. Commercial software; free trial. Also online checking.
http://elsop.com/linkscan/

A Real Validator
Windows. Based on an SGML parser. Shareware.
http://arealvalidator.com/

SP
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.
http://jclark.com/sp/

SP Instructions
Windows. Instructions for the Do-it-yourself Offline HTML Validator using SP.
http://ktmatu.com/info/do-it-yourself-offline-html-validator/

SWAN: SubWeb Analyzer
Unix. Generates statistics and finds inconistencies in a subweb (web site).
http://www.win.tue.nl/~wstomv/software/swan/swan.html

WebLight
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.
http://illumit.com/Products/weblight/