Voxel game-creation platform with easy modding and game creation
Magic Another Game Engine
Xash3D FWGS engine
Open source clone of the Age of Empires II engine
Python MUD/MUX/MUSH/MU* development system
Visual Basic 6 2D online adventure game engine
The multiplayer game engine
State Management and Multiplayer Networking for Turn-Based Games
Open source reimplementation of GTA San Andreas game engine in Unity
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
2D game engine for traditional platform games
Java decentralized cross-platform network multiplayer game engine
Free MMORPG Engine. Develop and host graphical browser-based games.