OpenSimulator is an open source multi-platform, multi-user 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. The optional Hypergrid allow users to visit other OpenSimulator installations across the web from their 'home' installation or grid.

OpenSimulator allows virtual world developers to customize their worlds using the technologies they feel work best - we've designed the framework to be easily extensible. OpenSimulator is written in C#, running both on Windows with the .NET Framework and on Unix-like machines with Mono. The source code is released under a BSD License, a commercially friendly license to embed OpenSimulator in products.

Git repo: git://opensimulator.org/git/opensim