I’ve been reading about real-time collaborative editing. One popular approach is OT (operational transformation). Most OT resources online reference this old paper on the Jupiter Collaboration system. It’s one of the first to introduce OT as a technique for building collaborative editors and while it’s easy to read, it’s light on details and spends more time than I’d like describing what real-time collaboration is (I guess you couldn’t take that for granted in 1995).
If you’re interested in OT, I’d recommend starting instead with this article by Daniel Spiewak. It describes OT at a high level and gets into some of the complexities of using it to build a server/client pair.
When I was done with Spiewak’s article, I still didn’t have a solid idea of how to implement OT on text or XML or any particular datatype. He directs to you to the Google Wave whitepapers to find out more, but I found the ot.py implementation to be a better explanation. It’s pretty small and easy to read in entirety. There are more implementations at that link (js, Haskell, lua) but I haven’t dug into them. If you find one that you think is great, let me know!