LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Glosbe - Huge collaborative massively multilingual dictionary.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
Visuwords - The fun and free way to look up definitions. THE graphical dictionary and visual thesaurus.
Gramlee.com - Online grammar check, proofreading and copy editing services.
Wordnik - the biggest online English dictionary, by number of words