There is opensim[1], which is used to run thousands of similar 3d worlds (shameless plug[2]), and the upcoming open source highfidelity platform [3].
1. http://opensimulator.org 2. http://opensimworld.com 3. http://highfidelity.io
1. http://opensimulator.org 2. http://opensimworld.com 3. http://highfidelity.io