Among many online dictionaries, this dictionary has a unique feature, when you have a question about the usage of the words, then you can ask it in the forum. Every time you look up some words, some forum links related to the word you're looking up are shown at the bottom of the definitions of each word.
This thesaurus is something, even someone like me, who have no interests in dictionaries, might be thrilled to watch the huge web of words. Sad thing is, if only it could come with a free limited version. You may try out three or four times of word search for free.