Grammarly - Clear, effective, mistake-free writing everywhere you type.
WriteNext - The writing application that boosts your writing. Increase focus and writing performance and reduce distractions by separating your creative writing from other activities. Let writing be the only activity you perform in the writing application.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Calmly Writer - Distraction-free text editor for Mac, Windows, Linux and ChromeOS.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Stashany - Online notepad for developers