Bartleby.com http://www.bartleby.com/ Definitions from the American Heritage Dictionary and Roget's Thesaurus. Very easy to use.
Cambridge Dictionaries Online http://dictionary.cambridge.org/ Choose from several dictionaries, including a Spanish/English and a French/English dictionary.
Hypertext Webster Interface http://www.bennetyee.org/http_webster.cgi A point-and-click client interface (for non-linemode browsers) for accessing the various Webster's dictionary services on the Internet.
One Look http://www.onelook.com Searches over 900 online dictionaries at once for a word definition.
Online Dictionary of the Social Sciences http://bitbucket.icaap.org/ Provides definitions for approximately 1000 entries in sociology, criminology, political science and women's studies. Focus is Canadian.
Oxford English Dictionary http://dictionary.oed.com/entrance.dtl A complete text of the 2nd. ed. of the Oxford English dictionary with quarterly updates, including revisions not available in any other form.
Foreign Languages
Diccionarios.com http://www.diccionarios.com/ The featured languages are Spanish and Catalan, but you can also search for French, German, and Portuguese language words.
WordReference.com http://www.wordreference.com/ French/English, Spanish/English, and Italian/English online dictionaries.
