Abe Fettig described JotSpot Live, what sounds like an AJAX real-time collaborative editor:

JotSpot Live is a real-time wiki interface that allows multiple users to edit the same page at the same time. Calling it “SubEthaEdit for the web” is an oversimplification, but that’s probably the quickest way to wrap your head around the concept. Unlike SubEthaEdit, JotSpot Live doesn’t require any special client software: it’s a web application that runs in Firefox, Internet Explorer, and Safari, using nothing but HTML, script, and CSS.

A few years back, I wanted to duplicate SubEthaEdit's functionality as a Java IDE plugin. If I recall correctly, SubEthaEdit has zero latency and locking because the clients communicate 100% asynchronously. When faced with edit collisions (i.e. I deleted a word you were editing), each SubEthaEdit client must generate the same resulting document regardless of the change ordering. The result: regardless of the performance of the network or other clients, collaborative editing in SubEthaEdit should respond as quickly as standalone editing. I moved on after realizing this would be a pretty tricky wheel to reinvent. I look forward to taking JotSpot Live for a spin. I'm most interested to see if it's 100% asynchronous like SubEthaEdit and how Abe balances the logic between Javascript and the server side.