GnuPlot - Gnuplot is a portable command-line driven interactive data and function plotting utility.
GNU Octave - GNU Octave is a programming language for scientific computing.
Giac/Xcas - Free computer algebra system
MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming
Geometry Pad - Geometry Pad is a dynamic geometry application for iPad and Android tablets.
Graphing Calculator - Visualize equations and functions in 2D, 3D and 4D.