Real Academia Española (RAE)  

Word Reference: The most useful section of Word Reference are the interactive forums in which translation queries can easily be solved by consulting the questions that have already been posted.

SpanishDict: It is a Spanish-English reference and learning website and mobile application. The website and mobile application feature a Spanish-English dictionary and translator, verb conjugation tables, pronunciation videos, and language lessons.

ProZ: Ex tensive translation dictionaries and glossaries for medical, legal, technical and other specialized terms, in Spanish, and many other languages.