NSZombieEnabled is an environment variable which controls whether the Foundation runtime will use zombies. When zombies are enabled, a deallocated object's class is dynamically changed to be _NSZombie, and by default, the memory region is never marked as free, although this can be controlled separately(so, remember to disable NSZombieEnabled for Archived Release Build).
The end result is that, with zombies enabled, messages to deallocated objects will no longer behave strangely or crash in difficult-to-understand ways, but will instead log a message and die in a predictable and debugger-breakpointable way. This is the tool to use when trying to track down over-releases and premature releases.
Zombies will take effect for all Objective-C objects that are deallocated through normal means, including most Cocoa classes as well as user-created classes. On 10.4 and earlier, a rather important exception to this is most/all TollFreeBridged classes, as they are deallocated using CoreFoundation which NSZombieEnab
