Syntensity, also known as the Syntensity Engine, and previously also called the Intensity Engine, is an open source project that consists of a client, server, and master (metadata) server – everything you need to run your own games or your own virtual world.
More specifically, it includes:
-3D rendering (from Cube 2)
-A large library of useful scripting code – weapons, movement options, items, events, etc.
-Physics (from Cube 2, currently working on adding Bullet physics as an option), both clientside and serverside (customizable)
-In-world editing, also in multiplayer (from Cube 2, with some additions)
-A plugin system, using Python, for extending the engine with additional components (plugins exist for saving highscore data locally using SQLite, etc., and experimental plugins exist for VLC, WebKit, Skype, etc.)
-An experimental browser plugin (for running the client in a web browser)
You can use the Syntensity Engine together with syntensity-com, which makes it easy to play new games and share your games with others. But you can also use the engine separately, either by running your own servers, or by running in an entirely local manner; for more details about those options, see README-standalone.txt in the source code.