Oxford English Dictionary [ http://www.askoxford.com/ ]
Search the Compact Oxford English Dictionary online; get answers to questions on English, language, grammar, spelling, quotations, and usage.

Cambrige Dictionary [ http://dictionary.cambridge.org/ ]
Publish dictionaries for people learning English all over the world.

The Free Dictionary [ http://www.thefreedictionary.com/ ]
Offers free access to an online dictionary, thesaurus and encyclopedia.

Dictionary.com [ http://dictionary.reference.com/ ]
Free online dictionary search, translator, word of the day, crossword puzzles and word games, and vocabulary learning resources for many languages.