It depends which parts of Mathematica you need.
Python is an alternative, especially within the Sage package [0].
Then there is also Maxima [1].
[0] http://www.sagemath.org/ [1] http://maxima.sourceforge.net/
Python is an alternative, especially within the Sage package [0].
Then there is also Maxima [1].
[0] http://www.sagemath.org/ [1] http://maxima.sourceforge.net/
Will check out Sage, haven't used Maxima since 2004. Thanks!
Edit: Sage looks nifty, Maxima looks unchanged, at least at screenshot-level.
Octave is more of an open source Matlab equivalent. That is, mostly linear algebra.