Google Translate - Google's free service instantly translates words, phrases, and web pages between English and over 100 other languages.
Glosbe - Huge collaborative massively multilingual dictionary.
Mate Translate - Ultimate translation app for Mac, iOS, Chrome and many more
Visuwords - The fun and free way to look up definitions. THE graphical dictionary and visual thesaurus.
Crowdin - Localize your product in a seamless way
Wordnik - the biggest online English dictionary, by number of words