@prologic@twtxt.net It was super useful if you needed to do the sorts of things it did. I’m pretty sad.
At its core was Sage, a computational mathematics system, and their own version of Jupyter notebooks. So, you could do all kinds of different math stuff in a notebook environment and share that with people. But on top of that, there was a chat system, a collaborative editing system, a course management system (so if you were teaching a class using it you could keep track of students, assignments, grades, that sort of thing), and a bunch of other stuff I never used. It all ran in a linux container with python/conda as a base, so you could also drop to a terminal, install stuff in the container, and run X11 applications in the same environment. I never taught a class with it but I used to use it semi-regularly to experiment with ideas.