Grammarly - Clear, effective, mistake-free writing everywhere you type.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
ProWritingAid - For the smarter writer. A grammar checker, style editor, and writing mentor in one package.
Ginger - Powerful and effortless desktop & mobile solutions for improving your writing and productivity. Ginger Software is your personalized editor - everywhere you go.
Typosaurus - Quickly check your website for spelling errors
Gramlee.com - Online grammar check, proofreading and copy editing services.