Mathematica is not really an "equation editor", it's more a
software package for performing symbolic mathematics.
Some open source initiatives that the OP might want to take a look
at:
Maxima
http://maxima.sourceforge.net/
Octave
http://www.octave.org/
Scilab
http://www-rocq.inria.fr/scilab/
The last 2 are more oriented towards numerical computation, so
they're more of a Matlab equivalent than a Mathematica equivalent.
I don't have any significant personal experience with these
packages, so give them a shot and let us know how they work out!
Regards,
Wald