IsoHill

Flixel

flixel is a free Actionscript (Flash) library that I distilled from a variety of Flash games that I've worked on over the last couple years, including Gravity Hook, Fathom and Canabalt. It's primary function is to provide some useful base classes that you can extend to make your own game objects.

FlashPunk

A free ActionScript 3 library designed for developing 2D Flash games. It provides you with a fast, clean framework to prototype and develop your games in. This means that most of the dirty work (timestep, animation, input, and collision to name a few) is already coded for you and ready to go, giving you more time and energy to concentrate on the design and testing of your game.

StarlingPunk

StarlingPunk is a framework built on top the Starling library designed to add structure and organization to your 2D game projects. It’s perfect for rapidly prototyping ideas and promotes code reuse between projects. As you may have been able to tell from the name, StarlingPunk is heavily inspired by the popular FlashPunk framework, although its not a direct port.

YCanvas

YCanvas is an open source 2D tile renderer library written in ActionScript 3. It provides high performance solution for rendering world maps. The library also contains Stage3D (GPU) accelerated implementation based on Starling.