TurboSphere
TurboSphere is an easy to learn and powerful JS based game engine.
..., and several have been implemented for TurboSphere's predecessor, Sphere (like the Majestic Map Engine (http://spheredev.org/smforums/index.php?topic=4496.0)).
TurboSphere is largely compatible with the Sphere RPG engine, the main API change is the replacement of object creating functions with proper JavaScript constructors. This means that there are several completed games already made that work with TurboSphere, requiring only a small set of compatibility scripts to function in TurboSphere.