Box2DFlash (aka Box2DAS3) is a port to AS3 of Box2D, an open source 2d physics engine by Erin Catto. It is mainly designed around rigid bodies and joints, but supports a host of other features, like raycasting, or continuous collision detection. You can use it to make games!
Currently we are nearing release 2.1, with an alpha release available, providing lots of API improvements and features, like the unlimited world size, one way collisions and a completely redone event system.
Go download the latest release! That contains the engine itself, and a testbed demonstrating how to get started. You could then have a read of the documentation, to see what features your missing out on. If you get stuck, try the FAQ, or ask on the forums. Try this list of useful articles too. Remember, Box2DFlash closely resembles Box2D, and so users from other languages will be able to help you.
You can try out the following links: