What Thesaurus websites do you folks find most useful?

My overall favorite is The Free Dictionary by Farlex:
http://www.thefreedictionary.com/manifold


Next best:

http://dictionary.reference.com/

http://www.websters-online-dictionary.org/


Less useful:

http://www.merriam-webster.com/

http://oxforddictionaries.com/

http://dictionary.cambridge.org/
-- 

Bob Stern