This is a very simple example showing how to easily implement multi-touch physics with Cocos2D 2.1 and Objective-Chipmunk.
Chipmunk (the C-API) is free and open source software. Chipmunk Pro (including the Objective-C binding and optimized solver) is what we've built on top of Chipmunk. Chipmunk Pro can help you save a lot of time as it plugs into the usual Objective C memory model and familiar APIs. It even works seamlessly with ARC. You can learn more here: http://chipmunk-physics.net/chipmunkPro.php
This example features the Objective-Chipmunk class ChipmunkMultiGrab, and it includes the trial version of Chipmunk Pro.