Take a peek at Visuwords - via a tip from the Nation Writing Project listServ:

Look up words to find their meanings and associations with other words and concepts. Produce diagrams reminiscent of a neural net. Learn how words associate. It's a dictionary! It's a thesaurus! Great for writers, journalists, students, teachers, and artists.The online dictionary is available wherever there’s an internet connection. No membership required. Visuwords™ uses Princeton University’s WordNet, an opensource database built by University students and language researchers. Combined with a visualization tool and user interface built from a combination of modern web technologies, Visuwords™ is available as a free resource to all patrons of the web.