Grammarly - Clear, effective, mistake-free writing everywhere you type.
tinySpell - A spell-checker for Windows. Checks text in clipboard and/or on the fly in any Windows program. Small and easy to use. Resides in the taskbar notification area. Free download.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
GNU Aspell - GNU Aspell, usually called just Aspell, is a free software spell checker designed to replace Ispell.
ProWritingAid - For the smarter writer. A grammar checker, style editor, and writing mentor in one package.