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.