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So far, I have found Tuxmath and Qmentat, but they have not been updated since 2018 and 2013, respectively. Geogebra and Sagemath are popular as well, but they seem to be more for teachers and advanced learners than for beginners.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.