Grammarly - Clear, effective, mistake-free writing everywhere you type.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
ProWritingAid - For the smarter writer. A grammar checker, style editor, and writing mentor in one package.
Gramlee.com - Online grammar check, proofreading and copy editing services.