The Tiny JavaScript Game Engine That Can!
Open source clone of the Age of Empires II engine
Magic Another Game Engine
Free, open source game engine online
Open-source engine for Heroes of Might and Magic III
Haskell game engine library for roguelike dungeon crawlers
Retro game engine for creating games like it's 1997
Open source reimplementation of GTA San Andreas game engine in Unity
Full featured MUD server with all the trimmings.
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
A mobile game distributed server implemented using netty4.X
Java decentralized cross-platform network multiplayer game engine
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3
Educational framework for OpenGLES, OpenCL and OpenAL
TR Game Engine allows you to develop your game quickly and easily.
A multiplatform game engine for interactive fiction experiences
Java Client + JS/Canvas Server-Side System, with maintained Objects
Cross-platform 2D game engine
Free MMORPG Engine. Develop and host graphical browser-based games.