3D game engine/framework in C, with Luajit and Python bindings now
A modern, cross-platform Quake game engine based on FitzQuake.
GemRB is a portable open-source implementation of the Infinity Engine
C++ lightweight cross-platform game engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
DOOM / Heretic / Hexen source port with enhanced graphics and UI
Opensource Game Studio
Portable game engine with main focus on point-and-click adventures
An advanced flexible game engine for Java.
Open Source 3D Game Engine
A 2D Adventure Game & Engine
A 3D, fully concurrent game engine
3D game engine for C and C++
Open source, moddable game engine for the "Polygon Love 2" game
The Lambda-Engine is a cross platform game engine.