Oxford English Dictionary: This 20-volume work (2nd ed) can be searched online together with revisions being made for the 3rd ed.(Subscription)
Cambridge Dictionaries: A selection of dictionaries published by Cambridges University Press can be searched; also provides links to sites for language teachers and researchers.
Collins Word Exchange: An online dictionary and discussion forum for words (many of them slang) used in everyday speech but not yet in dictionaries; includes a dictionary ofabbreviations used in mobile phone text messages and other word tools.
OneLook Dictionaries: Can be used to find definitions or translations of words.
Visual Dictionary Online: An all-in-one reference which provides images, words and definitions. Search the themes to quickly locate words, or find the meaning of a word by viewing theimage it represents. This dictionary from Merriam-Webster helps you learn English in a visual and accessible way.
Your Dictionary.com: Provides access to more than 1 800 dictionaries with more than 250 languages; its predecessor (A Web of Online Dictionaries), was launched in 1995 by Dr. RobertBeard at Bucknell University as a research tool for the world’s linguistic community but it is now a commercial venture.
Webster’s Online Dictionary: Contains at least 3,000,000 words and expressions from 90 modern and 10 ancient languages
WordReference.com: Translates words in French, Spanish and Italian. Beware of pop-ups