The WorldForge project is the original open-source MMORPG project, dating back to 1997.
- Crystal Space
Crystal Space is a mature, full-featured software development kit (SDK) providing real-time 3D graphics for applications such as games and virtual reality. It is free (LGPL) and cross-platform (Windows, GNU/Linux, Mac OS X).
FIFE stands for Flexible Isometric Free Engine and is a cross platform game creation framework. It provides you with the ability to create a game using Python interfaces.
The Gluon project is building a game engine and distribution system, which allows the creators of games to build games easily and collaboratively, powered by open technologies such as Qt, OpenGL, OpenAL, Git and many more.