online dictionaries

Monolingual dictionaries

Oxford University Press
http://www.oup.com/us/?view=usa 

Cambridge Advanced Learner's Dictionary

http://dictionary.cambridge.org 

MacMillan English Dictionaries

http://www.macmillandictionary.com

Online dictionary with definition, pronunciation and examples. (An American online dictionary and encyclopedia that gathers information from the The American Heritage Dictionary of the English Language, the Columbia Encyclopedia, the Computer Desktop Encyclopedia, the Hutchinson Encyclopedia and Wikipedia, as well as the Acronym Finder database, several financial dictionaries, legal dictionaries and other content )

http://www.thefreedictionary.com/squezze

Urban dictionary
(a web-based dictionary containing informal everyday language) 
http://www.urbandictionary.com/

English for Specific Purposes

Online dictionary for technical terms in various languages

http://iate.europa.eu/

Medical Dictionary
http://www.nlm.nih.gov/medlineplus/mplusdictionary.html

Bilingual dictionaries

An online translation dictionary for a wide variety of pairs (Greek included).

http://www.wordreference.com/