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.
Hemingway - Hemingway App makes your writing bold and clear.
Slick Write - Slick Write checks your writing's grammar and style for free!
Ginger - Powerful and effortless desktop & mobile solutions for improving your writing and productivity. Ginger Software is your personalized editor - everywhere you go.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.