How to build something similar to visual thesaurus

I would like to make a relational, dynamic cloud similar to the one used by Thinkmap, does anyone know what kind of code is used for these? Or if there are different ways of doing something like this, could I get a few pointers in the right direction? I'm not even sure what to call this sort of graph, so I'm having trouble with searches.

It uses java (you can tell because when you search it says 'starting java' and then loads a new window with a java applet.