.... Users can write programs in its native programming language, and many have contributed useful packages in a variety of areas over the decades.
Maxima is GPL-licensed and largely written in Common Lisp. Executables can be downloaded for Windows, Mac, Linux, and Android; source code is also available. An active community maintains and extends the system. Maxima is widely used.
Additional add-on packages for Maxima can be found at: https://github.com/maxima-project-on-github/maxima-packages