Is There a Free Alternative to Mathematica for Complex Calculations?

[JulieK]

I am trying to integrate a very messy expression using Wolfram online integrator but I get the message:
"You need to use Mathematica to evaluate this expression".
The online integrator works only for relatively simple expressions.
Unfortunately I do not have a license for Mathematica.
Any proposal to overcome this?
Is there a free computer algebra software that can be used as a replacement to Mathematica.

 

[fluidistic]

http://maxima.sourceforge.net/

 

[gsal]

Sage?

 

[Bill Simpson]

Axiom?

Any of these http://en.wikipedia.org/wiki/Computer_algebra_system

 

[Avichal]

SymPy