GeoGebra is a free mathematics software for learning and teaching. “GeoGebra is a multi-platform dynamic mathematics software for all levels of education that joins geometry, algebra, tables, graphing, statistics and calculus in one easy-to-use package. It has received several educational software awards in Europe and the USA.” -from their info page
A high school math teacher has created a lesson using Geogebra with the popular game Angry Birds as inspiration. You can find it here.