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.
None of the OpenSim grids is anywhere near as popular as Second Life. I sometimes still pop in to visit OSGrid, the most popular of the OpenSim-based virtual worlds, with about 65,000 user accounts created. I also visit InWorldz, which is not really considered a true OpenSim grid, because although it has its roots in OpenSim, Inworldz runs on the in-house developed Halcyon platform, while OSgrid runs on current OpenSim software.
There is a very active Google+ community called Opensim Virtual, where people post news and events happening in various OpenSim-based virtual worlds. The great thing about OpenSim is that there is nothing stopping you from setting up your own virtual world, which other users can visit! Many people have already done exactly that.
Thanks to Talla Adam for her comments urging me to include OpenSim on the list of virtual world platforms I have written about on the RyanSchultz.com blog!
UPDATE July 5th: Talla tells me that this is a more up-to-date list of OpenSim virtual worlds: Active OpenSim Grids (maintained by Hypergrid Business).