Mathics is a free, open-source 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.
mathicsscript: a text interface
For animated demos, see the mathics screenshot directory.
mathicsserver: a Django-based Web interface
Download and install Mathics
Wanted: Developers! Please get in touch if you want to be part of this awesome project.