Grammarly - Clear, effective, mistake-free writing everywhere you type.
Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
GNU Aspell - GNU Aspell, usually called just Aspell, is a free software spell checker designed to replace Ispell.
Enchant - The easiest way to scale personalized customer support
ProWritingAid - For the smarter writer. A grammar checker, style editor, and writing mentor in one package.