Simbody
Source code for the Simbody libraries, examples, and build system.
Simbody is a high-performance, open-source toolkit for simulation of articulated mechanisms, including biological structures, robots, vehicles, machines, etc. Simbody includes a multibody dynamics library for modeling motion in internal coordinates in O(n) time.
Read more about it here: https://simtk.org/home/simbody