W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
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.
Better Uptime - We call you when your website goes down