HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
Dood : The Puzzle Planet - A strategy puzzle game for the smart ones
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Trikaya - Atmospheric first-person puzzle game
Rocket Validator - Site-wide Accessibility and HTML validation made easy. With a single click, Rocket Validator will check thousands of web pages on your site for Accessibility and HTML conformance, producing a detailed, actionable report on seconds.
Kontrast - Check and adjust web element contrast in real time.