This is fun: Visuwords.
Visuwords™ online graphical dictionary — 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.
Enter words into the search box to look them up or double-click a node to expand the tree. Click and drag the background to pan around and use the mouse wheel to zoom. Hover over nodes to see the definition and click and drag individual nodes to move them around to help clarify connections. [continue, go play]
Found here at Metafilter.