IBDS - physics library
IBDS is a library for dynamic simulation of multi-body systems in C++. The library simulates rigid bodies, particles, many different joint types and collisions with dynamic and static friction. The library is open source under the zlib license. So it is free for commercial use.
IBDS has two meanings. It stands for Impulse-Based Dynamic Simulation and for Institut für Betriebs- und DialogSysteme which is the institute at the University of Karlsruhe where I developed the library.