Thanks! You're referring to the Sage notebook, and the key feature is that it is web-based with some collaboration support. I believe it was the first instance

Thanks! You're referring to the Sage notebook, and the key feature is that it is web-based with some collaboration support. I believe it was the first instance of this at the level of sophistication that it attained, which was significant given the web development tools available a decade ago. I started sagenb in 2006 after seeing a demo of a local OS X frontend for IPython. I did a huge amount of work on it, but there were also many students who did too (e.g., a 16-year old in the Philippines named Tim Dumol). The Sage notebook (and IPython) was very much inspired by Mathematica, Macsyma, and Emacs. Jupyter notebook was written a few years later, and has a more modern architecture (it used websockets). Jupyter has a much newer client called JupyterLab, which uses more modern Javascript tooling, including Typescript, WebPack, Phosphor, and so on. I've been working hard to make CoCalc fully support the Jupyter ecosystem, since I love the momentum behind it and how inclusive the community is. In particular, I wrote a new Jupyter client that supports realtime collaboration.