English Dictionaries/Diccionarios en Inglés

Cambridge Dictionary of American English

http://dictionary.cambridge.org/Default.asp?dict=A

http://dictionary.cambridge.org/Default.asp?dict=CALD 

Merriam Webster's Online Student Dictionary

http://www.wordcentral.com/

Merriam Webster's Visual Dictionary Online

http://visual.merriam-webster.com/

Bilingual Dictionaries

Word Reference

http://www.wordreference.com/