WORLDFORGE

World Forge is an open source game development engine, something I have always thought would be cool. They are laying the foundation for a customisable 3D game engine, ideally used for creating MMORPG's. I think the project has been running since 1998 or so, and they have released a couple of simple games using the engine.

The Idea is to release games using the same code and make each game more complex and complete. This is definately something to keep an eye on.