APE (Actionscript Physics Engine) is a free AS3 open source 2D physics engine for use in Flash and Flex. Some of the features:

• CircleParticles: fixed or non-fixed; variable mass, elasticity, and surface friction;
• RectangleParticles: rotatable; fixed or non-fixed; variable mass, elasticity, and surface friction;
• WheelParticles: traction and angular velocity; fixed or non-fixed, variable mass, elasticity, and surface friction;
• SpringConstraints: stiffness; collidable or non-collidible ,collidable constraints have variable width and scale;
• Grouping: composite class may contain Particles, and Constraints; group class may contain Particles, Constraints, and Composites;
• Collision: multisampling; static collision tests;