Grammarly - Clear, effective, mistake-free writing everywhere you type.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Grammar Snob - Markup your friends' iMessages like an English teacher 😁
Grammarian PRO3 - Grammarian PRO3 is a feature-rich grammar checker that enables you to write better content without taking assistance from the English teacher.
Deep Grammar - Grammar checker based on deep learning
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.