Wikipedia
RealXtend
realXtend is a project for creating an open-source virtual world platform which is intended to extend the capabilities of OpenSimulator to include features which are not available in the original Second Life project.
Like OpenSim, with which the project collaborates and from which it derives much of its code, realXtend is intended to allow individuals and groups to set up and browse their own virtual worlds without having to be connected to much larger worlds like Second Life.