Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
GNU Aspell - GNU Aspell, usually called just Aspell, is a free software spell checker designed to replace Ispell.
Slick Write - Slick Write checks your writing's grammar and style for free!
Enchant - The easiest way to scale personalized customer support