Physics & Collision detection Engine.

the Newton Dynamic engine, and Novodex (the star of the shows nowadays). Tokamak a bit under the weather.

I have used ODE with out any problems, I would suggest that you try out and see what you think.

IIRC Novedex is free to use for personal use, if you are going to make any money out of your project you have to buy the license for the engine.

I think there is also a list of free physics engines in the FAQ for the Physics & Maths Forum, you could do a bit of research on the ones on the list and then pick the one that suits your needs.

there's always tokamak. I've used it before. seemed pretty decent.

~guyaton