Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Slick Write - Slick Write checks your writing's grammar and style for free!
Grammatica - AI powered writing assistant
Gramlee.com - Online grammar check, proofreading and copy editing services.