You may have heard of Sage. It is a free open-source mathematics software system licensed under the GPL to create a viable free open source alternative to Magma, Maple, Mathematica and Matlab.
Sage is built on top of many existing open-source packages: NumPy, SciPy, matplotlib, Sympy, Maxima, GAP, FLINT, R etc. It accesses their combined power through a common, Python-based language or directly via interfaces or wrappers.
I thought since Mathematica already has its own specified area, it might be a good idea to create a stackexchane site for Sagemath as well.