Are there any great open-source implementations of OT? Last time I looked there were some, but the quality and popularity of them was questionable.
That guy also has a C library too. IIRC he used to work on the wave team.
ShareJS also supports collaboratively editing arbitrary JSON documents. Support for seeing everyone's cursors is being added at the moment - there's a sandbox kicking around here if you want to play with the prototype version: http://sharejs.org:7007/
However, for more complex tasks like rich text editing (at least richer than what markdown supports) you'll need to have an implementation tailored for the task.