Merriam Webster Dictionary - This one has a word of the day, dictionary and thesaurus along with other changing information.
Dictionary.com - is from ask.com and has fun and games and language resources.
YourDictionary.com this one is nice and simple and it can be followed on that new popular item, Twitter.
TheFreeDictionary.com is like some of the other sites but it has a fun hangman game and a spelling bee with different levels.
OnlineDictionary.com has deffinitions from the 1913 Webster dictionary and the WordNet Dictionary.
My favorite site is the WordNet Vocabulary Helper because it has definitions, synonyms, hypernyms and no ads or other things to distract.