For those in search of a FOSS alternative, give Mathics a try: www.mathics.org
In the past couple of months, Mathics had made quite a lot of progress.In particular the upcoming release will run on PyPy. Obviously it still had lots of catching up to do, but being written in python (and some Mathematica) makes development easy and fast. Come join us!
Disclaimer: Mathics vice dictator here