LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Clipboard Help+Spell - Clipboard Help+Spell is a text-based clipboard utility with the following features:
Grammarly - Clear, effective, mistake-free writing everywhere you type.
FreeSpell - FreeSpell is a spell checker for any and all Windows applications.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
ClipMem 3 - Advanced Clipboard-Manager