[jinlisp]

It depends of what you need, but for example for calculus is a nice program. There is also sympy and Wolfram Mathematica. For symbolic computation I think that Mathematica is the strongest then maxima and then sympy, but sympy is based on python and I think it will get stronger. If you need numerical computation then there is octave or matlab or julia.

[mrbungie]

For symbolic computation there is also Maple (*shudders at the thought*).