Scilab

Scilab is a free program for making numerical calculations. Scilab can do many of the same things as MATLAB can, but it does not cost anything.

Linux
Scilab is available through the package system in most Linux distributions. If it is not available for your distribution, you can find it on the Scilab project's website.
- Ubuntu and other Debian derivatives
  You can install Scilab by running this command:
  sudo aptitude install scilab
Windows
Download and install Scilab here.
OSX
Download and install Scilab here.
Please note that in order to install Scilab on OSX, you need an X11-server installed. There is a guide to installing X11 on OSX here, and for Mountain Lion here.