libbox2d is an opensource 2d physics engine.

Box2D is a feature rich 2d rigid body physics engine, written in C++ by Erin Catto. It has been used in many games, including Crayon Physics Deluxe, winner of the 2008 Independent Game Festival Grand Prize. Many ports are available for Flash, Java, C#, Python, JavaScript, D, and fixed point embedded devices.
You can get my compilable cmake source here : http://download.tuxfamily.org/gluon/Box2D/Box2D.new.tar.gz
My deb package here : http://download.tuxfamily.org/gluon/Box2D/libbox2d_2.0.1-1_i386.deb
The original code source : http://www.box2d.org/forum/download/file.php?id=365

WEBSITE : www.box2d.org