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.

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.

Just Not Sorry - Highlight 'weak' words so you can edit them out