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I mathematician friend referred me to a relatively new open-source project called Mathics. Since it uses Sage it seemed like it might be interesting for the group here. From the home page:
Mathics is a free, general-purpose online computer algebra system featuring Mathematica-compatible syntax and functions. It is backed by highly extensible Python code, relying on SymPy for most mathematical tasks and, optionally, Sage for more advanced stuff.