KDE Interactive Geometry Program (KIG) is a great tool for teaching math. Here is a good set of directions on how to do it.


If that article interests you, but you don’t use Linux, check out GeoGebra which is a program written in Java so it should run on any operating system that supports  Java Runtime 1.4.2 or later. You can even do a launch of the program from the Web.


[Geogebra link updated May 15, 2011]