• WorldForge
    The WorldForge project is the original open-source MMORPG project, dating back to 1997.
    http://www.worldforge.org (https://en.wikipedia.org/wiki/WorldForge)
  • 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).
    http://www.crystalspace3d.org
  • FIFE
    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.
    http://fifengine.net
  • Gluon
    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.
    http://gamingfreedom.org