Reacher - Check if an email exists without sending any email.
HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
mailfloss - Bulk email verification for busy businesses and email marketers. Put your email validation on autopilot. One-time, 60-second setup, no developers needed. Get your emails delivered to real people and maximize your email ROI.
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.