Sage: Open Source Mathematics Software
Sage is a free open-source mathematics software system licensed under the GPL. It combines the power of many existing open-source packages into a common Python-based interface.
Mission: Creating a viable free open source alternative to Magma, Maple, Mathematica and Matlab
Link to introductory videos on Sage and Python from the Sage website:
Screencasts and Videos - http://sagemath.org/help-video.html
Link to help and documentation from the Sage site: