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I have been looking for a good alternative to impactjs too. The one that I have been playing with is called Easel.js - http://easeljs.com/

i used to play around a lot with Akihabara (free & opensourced). The lack of a good map editor made me switch to ImpactJS

Suggest you to have a look at LimeJS (free & opensource). http://www.limejs.com

Have a look at craftyjs.com - not very mature, but moving fast, and really nice to work with.

I would suggest to take a good look at LimeJS.