Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
ispell - Ispell is a program that helps you to correct spelling and typographical errors in a file.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
DSpellCheck - Yet Another Notepad++ Spell-checking Plug-In
GNU Aspell - GNU Aspell, usually called just Aspell, is a free software spell checker designed to replace Ispell.