Dictionaries
Merriam-Webster Online
Features a dictionary and thesaurus, and extras such as Word of the Day and Wordgames.
Voycabulary
Takes a portion of text or websites and provides synonyms and dictionary definitions.
AskOxford
Contains a dictionary and thesaurus, as well as quotations, word of the day, games, and lexicography.
Encarta World English Dictionary
Provides audio clips of words pronounced.
American Heritage Dictionary
Over 90,000 entries feature 10,000 new words and senses, 70,000 audio word pronunciations, 900 full-page color illustrations, language notes and word-root appendixes.