Existing 3D Virtual World platform efforts:

• Second Life is the current leader of the pack with regards to number of users and public awareness. The frontend client was open sourced in early 2007, but we have yet to see significant user contribution. The backend is closed but will be opened at a later point.
• Project Wonderland is Sun's open source offering. Built on top of Project Looking Glass and Project Darkstar.
• Croquet is open source software development environment for the creation and large-scale distributed deployment of multi-user virtual 3D applications and metaverses that are persistent deeply collaborative, interconnected and interoperable.
• World of Warcraft is only the best known of the dozens and even hundreds of massively multiplayer online games - virtual worlds created around gameplay.