Grammarly - Clear, effective, mistake-free writing everywhere you type.
Glosbe - Huge collaborative massively multilingual dictionary.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Visuwords - The fun and free way to look up definitions. THE graphical dictionary and visual thesaurus.
Ginger - Powerful and effortless desktop & mobile solutions for improving your writing and productivity. Ginger Software is your personalized editor - everywhere you go.
Wordnik - the biggest online English dictionary, by number of words