[tromac]

GNU Octave is a basically identical language to MATLAB and, as the name suggests, is free. Identical to the point where most scripts will run using either platform without any issues or needing modification.

http://www.gnu.org/software/octave/

[baojieai]

+1, must also mention oct2py, the Python to GNU Octave bridge, that runs m-files from python. Some matlab legacy m-files may be rescued in this way.

Sage and IPython are alternatives, too. http://www.sagemath.org/ http://ipython.org/