Sage is mathematical software, very much in the same vein as MATLAB, MAGMA, Maple, and Mathematica. Unlike these systems, every component of Sage is GPL-compatible. The interpretative language of Sage is Python, a mainstream programming language. Use Sage for studying a huge range of mathematics, including algebra, calculus, elementary to very advanced number theory, cryptography, numerical computation, commutative algebra, group theory, combinatorics, graph theory, and exact linear algebra.
It is available for download from sagemath.org and its mirrors in source or binary form. If you have any questions and/or problems please report them to the Google groups sage-devel or sage-support. You can also drop by in #sagemath on freenode IRC.