Sorry to blow my own horn, but a few months ago we launched a similar project called Quolony https://quolony.com (we are a small startup from Salamanca, Spain). Right now we are working as a free service and we plan to publish it as open-source in the next few months. People can use it individually using a browser extension or in whole sites including our library. We have avatars, synchronized text selection, private and public chats, persistence, cobrowsing, file transfers and a lot more.
Some samples: https://quolony.com/demos/photomess/ (drag and drop) https://quolony.com/demos/quolony/developers/ (shared demo)
At the low level, we extend the browser event model to make it easier to use it in existing applications.
I hope you guys can give it a try and tell me what you think.
Kudos mozilla!
[EDITED: fixed linked]