The Tiny JavaScript Game Engine That Can!
Magic Another Game Engine
Full featured MUD server with all the trimmings.
Free, open source game engine online
The multiplayer game engine
3D playground built on three.js and cannon.js
A cross-platform 2D game engine
Educational framework for OpenGLES, OpenCL and OpenAL
Java Client + JS/Canvas Server-Side System, with maintained Objects
Free MMORPG Engine. Develop and host graphical browser-based games.