Cambridge Dictionary Online: http://dictionary.cambridge.org/
Longman Contemporary English Online: http://www.ldoceonline.com/
Oxford Dictionary Online: http://www.askoxford.com/
Dictionary.com: http://dictionary.reference.com/
Merriam - Webster Online: http://www.m-w.com/
Alpha Dictionary Online: http://www.alphadictionary.com/index.shtml
Lingvosoft Dictionary On Line: http://www.lingvosoft.com/English-Spanish-Dictionary/
ForeignwordDictionary: http://www.foreignword.com/Tools/dictsrch.asp?p=files/f_40_125.htm
FreeDictionary Online: http://www.freedict.com/
El Mundo.com Dictionary: http://www.elmundo.es/diccionarios/
LookWay up: http://lookwayup.com/free/SpanishEnglishDictionary.htm