Grammarly - Clear, effective, mistake-free writing everywhere you type.
ProWritingAid - For the smarter writer. A grammar checker, style editor, and writing mentor in one package.
Slick Write - Slick Write checks your writing's grammar and style for free!
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Hemingway - Hemingway App makes your writing bold and clear.