GnuPlot - Gnuplot is a portable command-line driven interactive data and function plotting utility.
GeoGebra Graphing Calculator - Interactive, free online graphing calculator from GeoGebra: graph functions, plot data, drag sliders, and much more!
Giac/Xcas - Free computer algebra system
Geometry Pad - Geometry Pad is a dynamic geometry application for iPad and Android tablets.
Dr. Geo - Dr. Geo, a software to design & manipulate interactive geometric sketches.
Graphing Calculator - Visualize equations and functions in 2D, 3D and 4D.