Current

• OpenSimulator (visit homepage)
  OpenSim is a virtual worlds platform, open source and under a nonrestrictive BSD license. It’s free to download and use and has gained some significant industry traction with patches and improvements from IBM, Intel, Microsoft and other companies.

Previous

• libsecondlife (visit homepage)
  libsecondlife is a protocol stack that allows you to communicate using the same language that the official Second Life™ viewer uses. libsecondlife is also used in OpenSim to allow you to connect to the server using the Second Life and RealXtend viewers.
• libterrain (no homepage)
  libterrain is a heightfield modification library designed to implement various erosion and procedural generation algorithms upon a 2D surface. libTerrain eventually got incorporated into the OpenSim project where it has morphed significantly. I may seperate this one out again some day if enough interest is shown in it.