LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Scribens - Scribens checks the grammar of your texts and finds spelling mistakes.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
WhiteSmoke - WhiteSmoke software products for writing and language. WhiteSmoke is a grammar checker, spell checker, style checker and more - in a single package. Also featuring translation software and English video courses.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Gramlee.com - Online grammar check, proofreading and copy editing services.