Box2DFlashAS3 is an open source port of Erin Catto's powerful c++ physics library Box2D.
Cycle through the demos above to see some of the features.
Full source code for the engine and examples can be downloaded from the project's sourceforge page found Here.
For further information check the ReadMe included with the download, post on the Box2D forums, or contact me at skatehead [at] gmail [dot] com.
Huge thanks to those who have supported this project with their generous donations:
Colin Northway (Fantastic Contraption)