engine.js plans to be a simple web-based game engine for multiplayer games that run off of Node.js and is built off of three.js which is an easy to use, lightweight, cross-browser, general purpose 3D library.
engine.js is a personal project and primarily being developed to serve as a game engine for another personal project (yet to be named).
I'll include more details as the project is developed but
in the meantime check out the Wiki here on GitHub.