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.
GrammarChecker.net - Grammar Checker is an online spell check tool. It effectively works on English sentence correction, Punctuation mistakes of 28 other different languages.
Enchant - The easiest way to scale personalized customer support
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.