OmniObjectMeter is an essential part of any Mac OS X developer’s toolbox: a tool that makes it easy for Cocoa and Unix developers to find and fix memory leaks, excessive allocation operations, “zombies”, prematurely deallocated objects, and other performance issues. The end product is faster and more robust applications.

http://www.omnigroup.com/developer/omniobjectmeter/

(Couldn’t you use Instruments for this?)