The Thinkmap Visual Thesaurus turns word-searching into meaning-mapping. Type in a word for which you want a synonym or antonym, and the program spits out a visualization of the connections based on linguistic meaning between the searched word and its corollaries. Synonyms are color-coded according to their grammatical type – verb, noun, adjective, adverb.
My immediate thought was how cool! And then I realized that full access to the thesaurus requires a subscription. Bummer.