Grammarly - Clear, effective, mistake-free writing everywhere you type.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
ProWritingAid - For the smarter writer. A grammar checker, style editor, and writing mentor in one package.
Grammarian PRO3 - Grammarian PRO3 is a feature-rich grammar checker that enables you to write better content without taking assistance from the English teacher.
Gramlee.com - Online grammar check, proofreading and copy editing services.