Orx: Portable Game Engine
GemRB is a portable open-source implementation of the Infinity Engine
Game engine supporting many 3D/2D formats and graphic effects
A fun realistic multiplayer free-and-open-source first-person-shooter!
Arianne is an engine to develop multiplayer online games like Stendhal
Low-Level Game Engine for Retro Platforms
Portable game engine with main focus on point-and-click adventures
SJCE - free portable cross-platform graphical chess game, 100% Java.
scriptable game engine with built-in support for 2D graphics and sound
Multi-platform game engine similar to old Megaman with a stage editor
plugin-based RPG manager in C++ and Qt5
A traditional top-down OpenGL car racing game
A generic 2D engine for any kind of game
Yet Another Chess ENgine
A Portable, open source, modular framework for playing various trading