Chipmunk is a high performance 2D physics engine that can really take maximum advantage of iOS hardware. This app is a physics toy that contains 15 interactive scenes that show off what Chipmunk can do.
Chipmunk is open source. You can download it and the source for this app from the Chipmunk Website. Chipmunk Pro contains extended functionality such as an Objective-C wrapper to take full advantage of iOS APIs and tools and also contains additional optimizations that allow it to take advantage of multi-core CPUs and the NEON vector coprocessor to get the best performance.