I mean one that transmits changes seamlessly in real time, not sharing a git repo or something like that
It always seemed to me that this could boost productivity of open-source projects build by volunteers all over the world quite a bit (if it isn't to distracting), but the actual implementations as some web-editors seemed always a bit gimmicky and not well integrated in people's work-flow.
Until it was abandoned in favour of a stand-alone project I thought this collection of Etherpad-lite plugins was the closest to something workable: http://codepad.etherpad.org as it included an option to push to a git repo on the same server. But I don't think anyone ever actually used that productively.
However, just now I got to know about the Saros project: https://github.com/saros-project/saros
Saros is an Open Source IDE plugin for distributed collaborative software development.
All participants of a Saros session have an identical local copy of the projects they work on.
They can work in their usual IDE using all of its features (currently only Eclipse, IntelliJ IDEA support is coming soon).
Saros keeps their copies in sync, so everyone can work completely concurrently and in real-time.
Learn more about Saros: http://www.saros-project.org/
Which is an open source plugin for the Eclipse IDE that seems to fully synchronise projects and even has a build in group-chat and whiteboard!
All it seems to require is a modern XMPP server backend (which I have now running quite well on xmpp.freegamedev.net in case you want to try it ).
I have not actually tested it... but this seems rather amazing
P.S.: don't miss their FAQ: http://www.saros-project.org/faq