Colbalt Calibur 3 is an real-time-ish multi-player MMO that uses Canvas and Node.js. It’s open source duel licensed under the BSD/GPL.
DivSugar is a CSS3 based 3D graphics engine. While it’s not as performant as a WebGL might be for full games, the fact that it uses CSS3 means that it can easily be integrated into existing sites and be used for casual games.
IvanK Lib is a HTML5 Engine inspired by Flash (the tag line is actually “Like Flash, but faster” that uses the power of GPU accelerated WebGL to render 2d graphics on the screen.
GameMaker HTML5 is an HTML5 port of the commercial GameMaker game building system currently in open beta.
RPG.js is a HTML5 Game Framework focused on building 2.5D Zelda Bitmap-based Role Playing Games.
WebGL framework for Data Visualization, Creative coding and Game Development
An open source beta JS Engine.
A modern, well-supported commercial HTML5 games engine.