LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
Gramlee.com - Online grammar check, proofreading and copy editing services.
DSpellCheck - Yet Another Notepad++ Spell-checking Plug-In
Grammarian PRO3 - Grammarian PRO3 is a feature-rich grammar checker that enables you to write better content without taking assistance from the English teacher.