Grammarly - Clear, effective, mistake-free writing everywhere you type.
EssayToolBox - EssayToolBox features lots of tools that checks any writing for grammar and spelling mistakes, detect plagiarism, brainstorms ideas, generates summaries and conclusion, counts words, and much more.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Write App - No-distraction writing tool for your productivity.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Slick Write - Slick Write checks your writing's grammar and style for free!