Maxima is the clone. I remember using it 20 years ago. very nice.
Maxima, despite its shortcomings, is a great tool for symbolic computation.
It's more like both Mathematica and Maxima are clones of the original MACSYMA.
Maxima is based on the DOE (Department of Energy) fork of MACSYMA.
Maxima is a similar CAS, but it's not a clone of Mathematica like Mathics aims to be.
This reminded me of one of the greatest posts in the history of the internet, and I am devastated to report that it no longer exists: https://news.ycombinator.com/item?id=6516114