Do you mean something like http://typewith.me but more specifically for code? I'm not sure of anything like that designed specifically for coding. You could use jsfiddle.net which is kind of like pastebin for front-end web development, but it's not real-time. There is also this service I've seen recently https://codeanywhere.net/ but it is not open-source. And then you could always share in Dropbox or use a private Github or run your own file server. But as for real time, I'm not sure of anything open-source like that.
I'm guessing the only real difference between what Etherpad (OpenSource engine behind typewith.me) does and what I would expect from a real-time coding platform is server-access to a project directory and syntax highlighting (and maybe some executable/preview function like on jsfiddle or ideone). So I guess you could add onto Etherpad for those features, but otherwise I don't really have a perfect open-source solution that comes to mind.
EDIT: I just found this one http://jsdo.it/ but I haven't experimented with it yet. It sounds a lot like jsfiddle but with real-time collaborating. Of course, you never mentioned which languages you are hoping to code in, so I might be going in the totally wrong direction with this.