Geogebra is a geometry program. It's free! Try it. I suggest using the offline installer, otherwise it downloads itself every time.

A guide tousing geogebra

Some of my geogebra files: load these into geogebra and see what they do.

Microsoft Mathematics is free and can be used like a graphical calculator, both for calculations and graphs. Unlike most calculators, it understands complex numbers.

Scilab is an open-source equivalent to the widely usedMATLABnumerical analysis package. It lets you plot experimental data, experiment with calculations, write programs and do design simulations.

Available for Windows, Mac and Linux.

RWM 2013