Kseg

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

KSEG is a Free (GPL) interactive geometry program for exploring Euclidean geometry. 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.

KSEG can be used in the classroom, for personal exploration of geometry, or for making high-quality figures for LATEX. It is very fast, stable, and the UI has been designed for efficiency and consistency. I can usually make a construction in KSEG in less than half the time it takes me to do it with similar programs.

KSEG was inspired by the Geometer's Sketchpad, but it goes beyond the functionality that Sketchpad provides.