Xash3D FWGS engine
The game engine for everyone
Voxel game-creation platform with easy modding and game creation
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
GemRB is a portable open-source implementation of the Infinity Engine
Orx: Portable Game Engine
2D Game Engine
Real-time RPG for PC/tablet/phone; multiple quests and random dungeons
online 4X browser game engine
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3
2D physics engine for games
Cross-platform 2D game engine