Total Validator - Java-based desktop tool validates HTML / XHTML / WCAG / Section 508 / Links / Spelling; free and pro versions. HTML validation against the W3C Specifications; accessibility validation against the WCAG (1.0 and 2.0) and US Section 508 standard; checks each page for broken links; spell checks (English, French, Italian, Spanish, German), 1-click validation using the Firefox/SeaMonkey extension, choice of browser identification to use when checking websites. Works on a wide varity of operating systems.
RealValidator - Shareware HTML validator based on SGML parser by Liam Quinn. Unicode-enabled, supports documents in virtually any language; supports XHTML 1.0, HTML 4.01, HTML 4.0, HTML 3.2, HTML 3.0, and HTML 2.0 ; extensible - add proprietary HTML DTDs or change the existing ones; fetches external DTDs by HTTP and caches them for faster validation; HTML 3.2 and HTML 4.0 references included as HTML Help. For Windows.
HTML Validator - Firefox add-on, open source by Marc Gueury. The validation is done on your local machine inside Firefox and Mozilla. Error count of an HTML page is seen as an icon in the status bar when browsing. Can validate the HTML sent by the server or the HTML in the memory (after Ajax execution). Error details available when viewing the HTML source of the page. Based on Tidy and OpenSP (SGML Parser). Available in 17 languages and for Windows and other platforms.
CSE 3310 HTML Validator - HTML syntax checker for Windows from AI Internet Solutions. Supports wide variety of standards; accessibility (508) checking; uppercase/lowercase converter. Free 'lite' version. For Windows.(Note: Many of the products listed in the Web Site Management Tools section include HTML validation capabilities.)