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