HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
Nibbler - Free tool for testing how good your website is, and what you can do to improve it. Check accessibility, SEO, social media, compliance and more.
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
FreePlagiarismChecker - Plagiarism scanner that detects plagiarism in your text.
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.
CSS Scan - Instantly check or copy computed CSS from any element for only ~95$