List of Online Dictionaries

Longman English Dictionary Online

Merriam-Webster English Dictionary and Thesaurus

Dictionary.com
American Heritage Dictionary of the English Language, Fourth Ed.

Cambridge Advanced Learner's Dictionary

Compact Oxford English Dictionary of Current English

Oxford Advanced Learner's Dictionary

Wicktionary
A collaborative project run by the Wikimedia Foundation.

WikIdioms
A collaborative dictionary of idioms.

Foldoc
A computing online dictionary.