Codenode: write Python and Sage in your web browser
codenode.org
codenode.org
In fact, I was one of the original creators of the Sage notebook (along with William Stein, Tom Boothby, and others).
We started Codenode with the hope of creating an even more generalized and robust online code notebook.
The benefits are more the ability to "code anywhere", to not have to install anything (think Windows here), to have the backend server contain lots of pre-installed packages, and to do real-time code writing collaboration with others.
The real-time collaboration is a work in progress. For more information, see here: http://github.com/clemesha/hotdot