The Plumb Design Visual Thesaurus is a language interface. The website is both an artistic exploration and a tool to explore, study, to analyse the structure of language.
It displays the interrelationships between words and meanings as spatial maps, and translates language into a visible architecture.
http://thesaurus.plumbdesign.com.
Plumb Design are also the makers of ThinkMap.
Update: ThinkMap’s Visual Thesaurus