Andrew Chamberlain is right: the Visual Thesaurus is quite possibly the coolest thing ever. In addition to being a weirdly beautiful and functional thesaurus in the conventional sense, it looks to have enormous potential as a brainstorming aide.
I can think of a few extensions of this form that would at least rival the thesaurus in coolness, however. Imagine if some service like Lexis-Nexis were to build a comparable tool for modelling and navigating interconnected news stories and legal opinions. One could trace the legal concept of, say, "privacy" as it weaves its way through case law and scholarly interpretation... or Monica Lewinski as she erupts into the zeitgeist and evolves from scandal to punchline.