Can someone familiar compare and contrast it with iPython Notebook (which also has support for any language/package that writes an ipython kernel plugin)
1) Beaker is language neutral from the ground-up, and not only can you mix languages, they can communicate. IPython does not have good support for multiple languages in the same notebook. And the backend is still pretty python-specific. Jupyter is working to address this, but I don't know how far along they are or how far they are going to take it.
2) Beaker UI has some nice new features: eg hierarchical sections that you can open and close, and one-click sharing.
3) Beaker is highly configurable and extensible via plug-in APIs so you or your organization can add your own languages, media types, security and authentication protocols, data sources, formats, menus, etc, and not have to worry about merging when a new version comes out.
4) Beaker has a great R backend.
5) Beaker is hiring! See http://www.twosigma.com/careers/position/936.html and http://www.twosigma.com/careers/position/935.html
compare:
1) Both have the same notebook and cell document metaphor.
2) Both are Open Source.
3) Both are great tools that I love.
Also, Beaker can import IPython notebooks, so it's easy to try it out and see the UI differences for yourself in the context of your actual work.