Grammarly - Clear, effective, mistake-free writing everywhere you type.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
DSpellCheck - Yet Another Notepad++ Spell-checking Plug-In
WhiteSmoke - WhiteSmoke software products for writing and language. WhiteSmoke is a grammar checker, spell checker, style checker and more - in a single package. Also featuring translation software and English video courses.
Grammarian PRO3 - Grammarian PRO3 is a feature-rich grammar checker that enables you to write better content without taking assistance from the English teacher.