Xash3D FWGS engine
Open source clone of the Age of Empires II engine
Open-source engine for Heroes of Might and Magic III
The Tiny JavaScript Game Engine That Can!
Voxel game-creation platform with easy modding and game creation
Magic Another Game Engine
Descent 3 by Outrage Entertainment
Free, open source game engine online
Full featured MUD server with all the trimmings.
Haskell game engine library for roguelike dungeon crawlers
The multiplayer game engine
State Management and Multiplayer Networking for Turn-Based Games
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
3D playground built on three.js and cannon.js
A cross-platform 2D game engine
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
A mobile game distributed server implemented using netty4.X
2D game engine for traditional platform games
Java decentralized cross-platform network multiplayer game engine
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3