This looks interesting:

“FlashPunk is a free ActionScript library designed for developing 2D Flash games. Its goal is to provide you with a fast, clean framework for prototyping and developing games; this means most of the dirty work — reliable framerate, sprite rendering/animation, player input, and collision detection (to name a few) — has been covered with a set of base classes and functions for your ease of use. This gives you more time and energy to concentrate on the design and testing of your game.”

As an alternative to Adam Saltsman’s Flixel, this certainly has potential. I’m not saying that Flixel needs a rival (I actually think it’s awesome), but competition is always good. Also, since both of them are open source, hopefully they’ll be able to learn from each other’s strong points.