Dr. Jan Bender has released a nice new open source physics simulator called IBDS. I'm fond of it because it is in the same vein of physics engine as the one in my Game Gems 4 article, but he's taken the concept much farther and provided a great many joint and constraint types. The most impressive thing about his new method is that difficult simulations normally solved with Lagrangian methods, such as the highly articulated tree mobile frequently seen in papers on the topic, execute extremely rapidly. Two scenarios that Dr. Bender's system handles very nicely (and which were not handled well in my system) are the cat's cradle executive toy, and chains. IBDS uses Erwin Couman's Bullet continuous-collision library.