What about http://mathics.github.io/?
I'm working on a fully featured jupyter kernel that will eventually get merged into the main Mathics repo [1]. I was planning to inquire about some hosting in a couple of weeks (once my exams are over and I have a chance to finish the jupyter kernel off).
It's analogous to the Octave/Matlab relationship.
The development team is also much smaller; I'm the only one contributing regularly to Mathics at the moment. Happy to answer questions.