Mathics maintainer here. Great to get everyone's feedback.
The page linked to is actually the old landing page. We have a new one here https://mathics.github.io/Mathics/ that is more up to date.
We're always looking for more developers/testers/designers etc. If anyone wants to get involved helping out feel free to get in contact with me
I used to have a HP-48, its symbolic integration functionality was more a parlor trick than something actually useful.
I would recommend Mathics, essentially a FOSS Mathematica clone written on top of the Python SymPy package: https://mathics.github.io/