Penflip - Typewrite is a real-time collaborative writing environment with Markdown support, versioning and Dropbox syncing. Alternatives to Penflip for Web, Windows, Mac, Linux, iPhone and more.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
Cleartext - A text editor that allows only the 1,000 most common words
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Quill - Powerful, API-driven rich text editor
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.