Mathics is described as 'general purpose computer algebra system (CAS) and designed to be a free, light-weight alternative to Mathematica. It's programmable using a Mathematica like language and uses a web interface for user interaction' and is an app in the Education & Reference category. There are more than 10 alternatives to Mathics for a variety of platforms, including Windows, Linux, Online / Web-based, Mac and Android. The best alternative is Mathematica. It's not free, so if you're looking for a free alternative, you could try SageMath or OpenAxiom. Other great apps like Mathics are PTC Mathcad (Paid), MathStudio (Paid), Cadabra (Free, Open Source) and Ascend (Free, Open Source).
SageMath is a free open-source mathematics software system licensed under the GPL. It combines the power of many existing open-source packages into a common Python-based interface. Mission: Creating a viable free open source alternative to Magma, Maple, Mathematica and Matlab.
ASCEND is a free open-source software program for solving small to very large mathematical models. ASCEND can solve systems of non-linear equations, linear and nonlinear optimisation problems, and dynamic systems expressed in the form of differential/algebraic equations.
SymbolicC++ uses C++ and object-oriented programming to develop a computer algebra system. Object-oriented programming is an approach to software design that is based on classes rather than procedures. This approach maximizes modularity and information hiding.