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.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
FreePlagiarismChecker - Plagiarism scanner that detects plagiarism in your text.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
CSS Scan - Instantly check or copy computed CSS from any element for only ~95$
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.