Dictionaries and Thesauri
American Heritage Dictionary : Also provides access to reference, verse, fiction, and nonfiction resources, including Roget's Thesaurus, Bartlett's Quotations, the King James Bible, Gray's Anatomy, and Bullfinch's Mythology.
BabelFish Translation
Merriam-Webster Online : Online dictionary containing an atlas, reverse dictionary, rhyming dictionary, thesaurus, and unabridged dictionary.
Roget's Thesaurus
yourDictionary.com : Access to more than 1800 dictionaries and translations for over 250 languages; also has links to glossaries, rhyming dictionaries, and thesauri.