The flexible game engine
The ioquake3 community effort to continue supporting/developing id's
Xash3D FWGS engine
3D game engine/framework in C, with Luajit and Python bindings now
Orx: Data-Driven Game Engine
Game engine to build text adventures.
A cross-platform, open-source, pure C game engine for mobile game
Retro game engine for creating games like it's 1997
Low-Level Game Engine for Retro Platforms
Game engine for playing tomb raider 1 - 5 levels.
A Simple Game Engine
3D game engine for C and C++
Tactical, turn-based game engine, that uses OpenGL and WinAPI
Reference implementation for the Doom game engine.