SageMath – Most powerful open-source math software on this planet

SageMath (previously called SAGE, short for System for Algebra and Geometry Experimentation) is a free and open-source mathematical software system for Windows, Mac and Linux platforms, developed at the University of Washington with over 100 mathematicians in 2005. It supports the research and teaching in algebra, geometry, number theory, cryptography, numerical computation, and related areas.