enchant.js:
is a framework for developing simple games and applications in HTML5 + JavaScript
was released in 2011, but has already had over 1,000 games and applications published
is open source (MIT license), and therefore free to use
has extensive supporting documents: API Documentation・Publications・Tutorial Site
can be augmented with various plugins to increase functionality
is continually developed and maintained by members of the Akihabara Research Center at UEI
is now on reddit!