>flixel is a completely free collection of Actionscript 3 files that helps organize, automate, and optimize Flash games; an object-oriented framework that lets anyone create original and complex games with thousands of objects on screen in just a few hours, without using any of the Flash libraries.
[Doesn’t require and actively advises AGAINST using Adobe Flash][link]. Lots of things happening in the 2d games space right now.
[link]: http://flixel.org/ “flixel”