Bullet is a professional open source multi-threaded 3D Collision Detection and
Rigid Body Dynamics Library. It is free for commercial use under the ZLib
license.

The library is being used by several professional game developers on PC,
PlayStation 3, XBox 360 and Nintendo Wii.

Sony Computer Entertainment provides a parallel SPU optimized version, and
collaborations with IBM, Intel, AMD and NVidia to exploit their parallel
hardware.

Bullet is native part of Blender 3D modeler and it supports COLLADA Physics
file format.

WWW: http://www.bulletphysics.com/