ENGLISH DICTIONARIES
Monolingual:
Cambridge Dictionary Online
Longman English Dictionary Online
Bilingual:
Wordreference.com (English-Castilian / Castilian-English)
Pronouncing dictionary:
Howjsay
Multilingual:
Diccionari de la llengua multilingüe (English-Catalan / Catalan-English)
Visual Dictionary:
Visual dictionary
Audiovisual Dictionary:
Languageguide.org
Etymological:
Dictionary.com