Merriam-Webster
Features free online dictionary, thesaurus, spanish-english and medical dictionaries, audio pronunciations, Word of the Day, word games, and many more language resources.
http://www.merriam-webster.com/

Your Dictionary
A free online dictionary that offers definition search from Webster's dictionary for word meanings and thesaurus. Use this dictionary search as a crossword or scrabble dictionary. Also find legal dictionary, technical words, glossary, foreign language translation, medical dictionary, and rhyming dictionary lists.
http://www.yourdictionary.com/