Frankenstein lived
0 A.D. is a free, open-source, cross-platform real-time strategy game.
Web powered cross-platform 3D, WebXR game engine
Massively multiplayer online game server and client framework
Android, DOS, FirefoxOS, Linux, MacOS and Windows all in one..
Free open source game created in Unity 3d for Windows 64 bits
An extensible development framework for roleplay games.