In my recent talks I always give realtime collaborative code editing as a great example of collaboration. As far as I know Cloud9 are the first to add this features to an online IDE (I've heard such a feature has been available in desktop IDEs for a while).
They use socket.io for their solution, which isn't a surprise since they're big on node.js.
The recent mail-out must have really driven some traffic they're way as they've reported:
We are experiencing the highest demand in our history. If you are having issues with your workspace, we are fixing issues now. Sit tight!
— Cloud9 IDE (@Cloud9IDE) June 27, 2012
I wonder how much this increase in traffic was influence by this new awesome realtime functionality?