"Sage is a huge distribution of free open
source mathematical software:

Algebra and calculus: Maxima, SymPy

High precision arithmetic: GMP, MPFR, MPFI, quaddouble, Givaro

Commutative algebra: Singular

Number theory: PARI, NTL, mwrank, ECM, FLINTQS, GMP-ECM

Exact linear algebra: LinBox, IML

Group theory: GAP

Scientific computation: GSL, SciPy, NumPy, cvxopt

Statistical computation: R

Graphics (2d and 3d): Matplotlib, Tachyon3d, Jmol

Sage has a highly developed unified collection of pseudo-tty based
interfaces that make it is possible to make extensive use of Maple,
Mathematica, Magma, Matlab, GAP, Maxima, Singular, PARI, and many other
systems from anywhere within a single Sage program." It is available for
Linux, Mac OS X and Windows or you can download the source code.

Check
here
for a short overview or check Sage webpage
for more.