http://flashpunk.net/?p=news

Haven't used this myself, but this is from the website.

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.

It's important to note that FlashPunk is targeted towards the development of games with 2D raster/bitmap graphics, as opposed to vector graphics. It can manage thousands of animated bitmap sprites on-screen at a time without slowing, a lot faster than Flash normally could, because it operates under the assumption that your game primarily uses bitmapped graphics.