TheRightMargin - TheRightMargin is a smart writing tool.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Writefull - Writefull provides you with the tools you need to improve the quality and consistency of your writing.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
Analyze My Writing - Analyze My Writing is an online text content and readability analyzer.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.