I have created a application with D3D9 and I readed list of physics engine. But, I have any question:
- Which free physics engines is better for my application?
- Which is the simplest?
- Which you advise?
Sorry for my English and thanks for replys.

Which free physics engines is better for my application?

Well, what is your application ?

- Which you advise?

I would sugguest bullet .

My application is a game engine.

I tried bullet and PhysX, and I find PhysX easier to use personally.

Ok thanks you, I going to use PhysX, but I will try also bullet because it's free.

PhysX binaries are also free for some platforms - you should check.

Bullet has lots of great functionality and can be made to work... but (having used it and PhysX) it has pretty bad documentation and a very messy API (sorry Erwin!). PhysX has got very good docs, even more functionality, and a super clean API (and getting better all the time - 3.2.0 is just out and is great). On the other hand (if I remember correctly), there was a post a little while on the Bullet forums indicating Bullet has got smaller overhead in the simulate call, but in practice that's probably irrelevant. Bullet forums are pretty good, which luckily makes up for the documentation quite a lot.

So - both (and others) are worth considering, but it depends on whom you're targeting. I'm using Bullet in my (pet) project, and have no problems with what it does: https://play.google.com/store/apps/details?id=com.rowlhouse.picasim