GemRB is a portable open-source implementation of the Infinity Engine
Game engine supporting many 3D/2D formats and graphic effects
RTSish Game Engine
ZDoom 2.8.1 fork focusing on modernisation, performance and stability
The world is no longer the same
Amateur 3D Game Engine
Orx: Portable Game Engine
An isometric arcade-style game of skill, puzzles and exploration
Ze Text Adventure game Builder : Create as you play !
C++ lightweight cross-platform game engine
Vivid3D is an open-source 3D engine with powerful 2D UI.
A fork of Chocolate Doom with small Quality of Life enhancements
A fork of dhewm3 with an emphasis on modernisation and vanilla Doom 3
a UCI chess engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Vivid is a modern C++ 3D engine using OpenGL4+
Go 3D Game Engine
DOOM / Heretic / Hexen source port with enhanced graphics and UI
2D Game Engine