Open source clone of the Age of Empires II engine
Retro game engine for creating games like it's 1997
Open source reimplementation of GTA San Andreas game engine in Unity
DOOM / Heretic / Hexen source port with enhanced graphics and UI
A fast, scalable, distributed game server engine/framework for C++
Game engine for playing tomb raider 1 - 5 levels.
A 2D Adventure Game & Engine
TurboSphere is an easy to learn and powerful JS based game engine.
A retro style video game engine for RPGs and adventure games.
Cross-platform 2D game engine