Open Croquet

Croquet is a powerful new open source software development environment for creating and deploying deeply collaborative multi-user online applications on multiple operating systems and devices. Derived from Squeak, it features a peer-based network architecture that supports communication, collaboration, resource sharing, and synchronous computation between multiple users on multiple devices. Using Croquet, software developers can create and link powerful and highly collaborative cross-platform multi-user 2D and 3D applications and simulations - making possible the distributed deployment of very large scale, richly featured and interlinked virtual environments.

http://www.opencroquet.org