Sage is an open source collection of mathematical software with a common Python interface. The lead developer is William Stein, a number theorist in the Mathematics Department at the University of Washington.
The Sage notebook was an original model for the IPython_notebook.
You can try Sage online by typing into a cell at https://sagecell.sagemath.org/. You can type straight Python as well as using the enhanced capabilities of Sage.
See:
for some online sage examples.