SAGE

SAGE is free open source math software that supports research and teaching in algebra, geometry, number theory, cryptography, numerical computation, and related areas. Both the SAGE development model and the technology in SAGE itself is distinguished by an extremely strong emphasis on openness, community, cooperation, and collaboration: we are building the car, not reinventing the wheel. The overall goal of SAGE is to create a viable free open source alternative to Maple, Mathematica, Magma, and MATLAB.
Find SAGE at: http://sagemath.org/