OpenSim is a Free and Portable Virtual Reality World Server. It can be used to create Simulated Virtual Worlds similar to those in SecondLife, but self hosted. OpenSim is written in C#, ships with a pre-configured sqlite database. It is also self contained and ready to run out of the box. Client connection requires that a port is opened and that the client connect via a viewer such as Hippo Viewer.
OpenSim - A Free Portable Virtual World Server
Authors Website: http://opensimulator.org/wiki/Main_Page
Extracted Size: 65 MB
License: Open Source/BSD License
Download: Get this Portable Virtual World Server Simulator HERE