Open-source physics engines (2011)
ibm.com
ibm.com
http://elib.dlr.de/79462/1/74320346.pdf
The authors, who were already using Bullet, concluded that "Newton and PhysX would be valuable candidates to compete with Bullet for integration in our current simulation environment."
C++'s generally poor interoperability has been annoying me since the 90s. :-(