Physics Abstraction Layer
Open source, free for commercial use. Can select which engine you would like to use from ode, newton, physx, tokamak, true axis, bullet, jiglib, and open tissue.
Tokamak Physics Engine is now released as Open Source Software under the BSD license.
Tutorial at Irrlicht Homepage
Free for commercial use, including PlayStation 3, Open Source multiplatform C++ under the Zlib License.
Quote from the website:
"Box2D is an open source physics engine written primarily for games. As the name suggests, Box2D is a purely 2D engine. Box2D can now handle convex polygons and other shapes coming soon.
Box2D falls under the permissive zlib License."