Grammarly - Clear, effective, mistake-free writing everywhere you type.
StoryMill - Developers and publishers of Mac, Win, and iOS productivity, home and office and writing software such as MacGourmet, Paperless and many more. Based in MN.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
BlankPage - Stay motivated to write, and finish your story
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Manuskript - Open-source tool for writers.