Visual Thesaurus is a pretty cool graphic tool that lets you explore new words. Unfortunately you can only demo it for a short period before the site asks you to buy the premium service...
In short, this is both a dictionary and a thesaurus that lets you explore words using a branch like structure. As the site describes, it is, "a marvelous way to improve your vocabulary and your understanding of the English language."
Visual Thesaurus: Innovative New Tool
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