Online Dictionaries

Cambridge Dictionary

http://dictionary.cambridge.org/

Macmillan Dictionary

http://www.macmillandictionary.com/

Oxford Learner Dictionary

http://oald8.oxfordlearnersdictionaries.com/

A multimedia pronunciation dictionary

http://www.embedplus.com/dictionary/how-to-pronounce-words-and-use-them.aspx

 You can search for a word and get not only authentic audio pronunciations but also videos of people speaking and using the word in context.