KSEG is a Free (GPL) interactive geometry program for exploring
Euclidean geometry. It runs on Unix-based platforms (according to users,
it also compiles and runs on Mac OS X and should run on anything that Qt
supports). You create a construction, such as a triangle with a circumcenter,
and then, as you drag verteces of the triangle, you can see the circumcenter
moving in real time.

Homepage:
http://www.mit.edu/~ibaran/kseg.html
