OpenSimulator is a multi-platform, multi-user 3D distributed virtual environment platform. Out of the box, it can be used to simulate virtual environments similar to that of Second Life. These can be used as social virtual worlds or for specific applications such as education, training, and visualization. Access is via the regular Second Life open-source viewer or via third-party clients. There are a number of private and public deployments of OpenSimulator, including OSgrid, which has over 8000 regions hosted by independent individuals and organizations spread over the Internet.
Aurora-Sim is the next generation of the OpenSimulator Project. It allows you to run a large scale operation and extend it as needed, while still having many features available for your use. The architecture is designed to be as flexible as possible so that third parties can create modules by plugging into the modular design. It is cross-platform compatible by using the .NET framework and Mono. Most importantly, it is a community based project consisting of individuals with a variety of talents (technical and non-technical) and has been developed with an emphasis on security, speed, and performance.