OpenSim is a 3D Application Server. It can be used to create a virtual environment (or world) which can be accessed through a variety of clients, on multiple protocols. OpenSim allows you to develop your environment using the technologies you feel work best - we've designed the software to be easily extendable through loadable modules to build completely custom configurations. OpenSim is released under a BSD License, making it both open source, and commercially friendly to embed in products.

For more information visit the OpenSim main home page at http://opensimulator.org