One thing that I found useful was poking my head around Erlang and RabbitMQ. It's a messaging system based on Erlang. You could also check out PubSubHubbub for

One thing that I found useful was poking my head around Erlang and RabbitMQ. It's a messaging system based on Erlang. You could also check out PubSubHubbub for push notifications which is probably similar to what you're looking for. As someone had mention, Etherpad is out there and soon Google Wave. For a Twitter clone, there's Statusnet.ca