Maxima which is a direct decedent( I think they even share code now) of Macsyma one of the first symbolic math packages developed at MIT. Wolfram was inspired by it when he created Mathematica. In fact I think it was because he despised macsyma (cause it was in lisp and not C I believe, there are some mailing list post describing the events) that he decided to create Mathematica. So the enemy of your enemy is your friend ;p
You might try sympy (a python project)--the community is very responsive. But, be prepared to get your hands dirty if there are some features that aren't present yet.
