SageMathCloud (https://cloud.sagemath.com) is a completely open source (https://github.com/sagemathinc/smc/tree/rethinkdb) collaborative code editing environment built on CodeMirror. I love CodeMirror and massively appreciate marijn's work on the project, support, leadership, etc. I implemented Neil Fraser's differential sync (https://neil.fraser.name/writing/sync/) algorithm in it (back in 2013), and have refined it little by little since. The core sync code is (https://github.com/sagemathinc/smc/blob/rethinkdb/salvus/dif...), and is BSD licensed. All development of SageMathCloud is done in the open (https://github.com/sagemathinc/smc/commits/rethinkdb), and it's very active right now. Our next big release, which should be within the week, involves rewriting the backend to use RethinkDB (instead of Cassandra), and the frontend to use React.js + Flux heavily (instead of jQuery).