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For my (limited) needs, I’ve used a number of math packages, namely bc, octave and numerical python for calculations, Yacas for symbolic computations, Gnuplot, Metapost and recently visual python for graphs, and GMM++ for matrix and vector calculations in C++. None of these tools is entirely satisfactory, but combined, they usually got the job done. It’s just annoying that each and every one of them uses its own language which you have to learn before you can use it. (Not counting the python and C++ libraries, of course.)
Other people use R, the swiss army chainsaw of mathematics, for everything. So far, it has been too intimidating for me to learn.
One suggestion from the Slashdot thread that I will have to check out is Maxima: It’s a symbolic computation package derived from an MIT project from the 70s called Macsyma. I don’t know whether it will be of much use, since apparently it hasn’t been under active development (just maintenance) since 1982.