GeoGebra is free and multi-platform dynamic mathematics software for learning and teaching geometry, algebra, statistics, and calculus. Download the free software at http://www.geogebra.org. Find the source code at http://www.geogebra.org/trac
UPDATE This version of the software is very out of date and will be replaced by a much smaller UI along with much more optimizations like the use of Directx. Almost complete, just need to research Lame OGG and FLAC encoders before it is finished!