Java Object System
Overview - for my use
ODE - Used all over the place, including nebula/crystal space. What is probably the best choice. It follows nebula's odd naming conventions () ..so it must be super cool.
Dynamechs - Free - Designed for robotics simulations (UNDERWATER) - Looks slow thoughFastCar - $20000 - Vehicle physics library from oxford - demo is fun, try it, free limited version
Also has a free multi-object library available
Vortex - Free trial available, seems defense industry oriented i.e. antiquated
Kano's Rigid body - pretty cool, has a limited set of shapes, but otherwise sweet, fun to play with, no library though. GET THE DEMO
Evolved Robots - Uses ODE physics