Cambridge Dictionaries
Offers an online dictionary and sells a variety of dictionary products.
http://dictionary.cambridge.org/
Cool Dictionary
Webster-based dictionary with pronunciation audios. Also makes use of Wiktionary and Wikipedia resources.
http://www.cooldictionary.com/
Dictionary Link
Guide to free online dictionaries and translation services. Also provides links to thesauri and encyclopedias.
http://www.dictionarylink.com/
ESL Dictionaries and References
Selection of detachable and free resources for learning the English language. With direct links to other online dictionaries.
http://www.esldesk.com/dictionaries/