Xash3D FWGS engine
Open source clone of the Age of Empires II engine
Voxel game-creation platform with easy modding and game creation
Magic Another Game Engine
The Tiny JavaScript Game Engine That Can!
Open-source engine for Heroes of Might and Magic III
Free, open source game engine online
Descent 3 by Outrage Entertainment
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