Mathics is a general-purpose computer algebra system (CAS). It is an open-source alternative to Mathematica. It is free both as in "free beer" and as in "freedom".
The home page of Mathics is https://mathics.org.
See the wiki page for how to install and run.
Please feel encouraged to contribute to Mathics! Create your own fork, make the desired changes, commit, and make a pull request.
Mathics is released under the GNU General Public License Version 3 (GPL3).