W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
The Tweet Hereafter - The last tweets of the deceased
Total Validator - Total Validator can check your website to help ensure that it is accessible, uses valid HTML and CSS, has no broken links, and is free from spelling mistakes.
Your Life In Weeks - A spreadsheet of your life inspired by Wait But Why
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.
Startup Graveyard - History shouldn't have to repeat itself