LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
SSuite Spell Checker - SSuite Office's free spell checker provides you with a handy means of checking the spelling in order to make sure your words are correctly typed or written.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
Ginger - Powerful and effortless desktop & mobile solutions for improving your writing and productivity. Ginger Software is your personalized editor - everywhere you go.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Clipboard Help+Spell - Clipboard Help+Spell is a text-based clipboard utility with the following features: