Croquet is a powerful open source software development environment for the creation and large-scale distributed deployment of multi-user virtual 3D applications and metaverses that are (1) persistent (2) deeply collaborative, (3) interconnected and (4) interoperable. The Croquet architecture supports synchronous communication, collaboration, resource sharing and computation among large numbers of users on multiple platforms and multiple devices.
I don't have much time to try it, but it seems pretty interesting : The Croquet Consortium
Yes, I have been experimenting with this, Alice In Wonderland, Cobalt, OpenSim, RealXtend etc etc, but, cant help but feel Atmos kicked all these into orbit...