Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
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.
Manuskript - Open-source tool for writers.
Grammarian PRO3 - Grammarian PRO3 is a feature-rich grammar checker that enables you to write better content without taking assistance from the English teacher.
Novelize - Novelize is an online novel writing app designed to keep authors on track.