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.
My words on Stories page:
Sage is one of the well designed approaches to blur the boundaries among being a user, developer and contributor in the open-source ecosystem. Probably I will be hanging around for a long time be it bugging, complaining, documenting, adding content and eventually digging into source codes ;)