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.
tota11y - An accessibility visualization toolkit from Khan Academy
FreePlagiarismChecker - Plagiarism scanner that detects plagiarism in your text.
CSS Scan - Instantly check or copy computed CSS from any element for only ~95$
Page Speaker - Modern web accessibility tools for agencies & webmasters
accessiBe - Making websites accessible to people with disabilities