American Heritage Dictionary of the English Language, 4th ed. - American Heritage does not have its own site but is available at Bartleby.  The researcher will find more than 90,000 entries, 70,000 audio pronunciations, and 900 color illustrations.  Besides entry word and full text, several types of additional searches are available, including definition, etymology, and articles.  

Medical Dictionary - MedTerms - A gold star site rated the best medical dictionary on-line

Merriam Webster's Word Central Site

Roget's Thesaurus II : the New Thesaurus

One Look Dictionaries - The Fast Finder - Indexes 750 dictionaries and more than four million words.  Users can select individual titles or search the entire database.

Pseudodictionary: the dictionary for words that wouldn't make it into dictionaries - anyone can submit a term to this slang dictionary, which currently consists of almost 10,000 words.  

Web Thesaurus Compendium

Your Dictionary - yourDictionary.com provides the most comprehensive and authoritative portal for language and language-related products and services on the web with more than 1800 dictionaries with more than 250 languages. More than 1,500,000 people a month visit YDC.

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Acronym Finder - A terrific shortcut if you don't care to browse but just want to find out what WTF means.