GemRB is a portable open-source implementation of the Infinity Engine
Game/Scene Development System
ZDoom 2.8.1 fork focusing on modernisation, performance and stability
RTSish Game Engine
A free 3D game engine to create games and realtimes scenes for Windows
A fork of dhewm3 with an emphasis on modernisation and vanilla Doom 3
DOOM / Heretic / Hexen source port with enhanced graphics and UI
A cross platform Lua based game engine
Paintown is a 2d fighting game engine.
Open Source 3D Game Engine
A 2D game engine base on OpenGL and SDL,support for Windows & Linux
a flexible game engine
Tactical, turn-based game engine, that uses OpenGL and WinAPI
WGFX 2D Game Engine