Open source clone of the Age of Empires II engine
The multiplayer game 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++
Planimeter Game Engine 2D - LÖVE-based game engine for Lua
Multiplayer game server based on Node.JS
Java decentralized cross-platform network multiplayer game engine
A retro style video game engine for RPGs and adventure games.
Java Client + JS/Canvas Server-Side System, with maintained Objects
Cross-platform 2D game engine