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
Ro3n is a simple Qt application designed to solve mathematical equations - linear as well as quadratic.
Vofce is a simple GUI driven graph plotting tool mainly for math students.