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.
Ginger - Powerful and effortless desktop & mobile solutions for improving your writing and productivity. Ginger Software is your personalized editor - everywhere you go.
DSpellCheck - Yet Another Notepad++ Spell-checking Plug-In
Gramlee.com - Online grammar check, proofreading and copy editing services.