Grammarly - Clear, effective, mistake-free writing everywhere you type.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Expresso - The award-winning Expresso editor is equally suitable as a teaching tool for the beginning user of regular expressions or as a full-featured development environment for the experienced programmer with an extensive knowledge of regular expressions.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Typely - A sensitive, unobtrusive and reliable tool for any writer, newspaper editor, teacher, blogger or student STA
ProWritingAid - For the smarter writer. A grammar checker, style editor, and writing mentor in one package.