Validator-Lite is a simple but rigorously-correct validator for HTML, XHTML, and other DTD-based XML languages. Its basic purpose is to offer a bare-bones GUI for users who find "raw" OpenSP tools such as nsgmls difficult or confusing. It is a lighter weight tool than the webserver- based validators, and by installing everything locally it runs faster, and works when you're offline.

For more information, please see the help page, the download/install page, and a screenshot.