http://opensimulator.org/ from the site :

What is OpenSimulator?

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. It also has an optional facility (the Hypergrid) to allow users to visit other OpenSimulator installations across the web from their ‘home’ OpenSimulator installation. In this way, it is the basis of a nascent distributed Metaverse.

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 over the .NET Framework and on Unix-like machines over the Mono framework. The source code is released under a BSD License, a commercially friendly license to embed OpenSimulator in products. If you want to know about our development history, see History.

Out of the box, OpenSimulator can be used to simulate virtual environments similar to Second Life™, given that it supports the core of SL’s messaging protocol. As such, these virtual worlds can be accessed with the regular SL viewers. However, OpenSimulator does not aim to become a clone of the Second Life server platform. Rather, the project aims to enable innovative feature development for virtual environments and the Metaverse at large.

OpenSimulator is getting more stable over time but is still a high complex software system that can suffer various bugs and quirks; handle with care!

More to come…