I have been playing around with the Motor2 physics engine which is based on the Box2d engine. I really like this engine and think it has a lot of potential for actionscript game developers. Box2D has great documentation while Motor2 does not have any documentation at all. My advise when you start to work with the Motor2 engine is to have the class files rather than the swc files in the project. This will help you get a look into the code to check the parameters and methods.

Google Code
http://code.google.com/p/motor2/

Google Groups
http://groups.google.com/group/motor2

Box2D Documentation (may help)
http://www.box2d.org/manual.html