Singular is a computer algebra system for polynomial computations, with special emphasis on commutative and non-commutative algebra, algebraic geometry, and singularity theory. It is free and open-source under the GNU General Public Licence (See COPYING).

Installation instructions from source code: https://github.com/Singular/Singular/wiki/Step-by-Step-Installation-Instructions-for-Singular
Homepage: https://www.singular.uni-kl.de/
Source code: https://github.com/Singular/Singular (git repository)

ftp://jim.mathematik.uni-kl.de/pub/Math/Singular/src (tar balls) https://www.singular.uni-kl.de/ftp/pub/Math/Singular/src (tar balls)
User Manual: https://www.singular.uni-kl.de/Manual/latest
Bug/Issue-Tracker: https://www.singular.uni-kl.de:8002/trac
Singular Wiki: https://github.com/Singular/Singular/wiki/
Developer and Reference Manual: https://www.singular.uni-kl.de/dox/html