ZoneMonitor, Mystery Tool
If you look at /Developer/Applications/Utilities/ZoneMonitor.app, you’ll find a garbage collection debugging tool. Apple’s documentation refers to it as an “application for analyzing memory usage” but doesn’t say any more.
I had to launch it twice—the first time it crashed. I don’t know if that was a fluke or not (could have been). On second launch I pointed it at a little demo garbage-collected app, and it launched collected and non-collected versions and compared them. I double-clicked on auto_zone, and got a new window with tabs for Heap and Roots. Etc. It’s explorable—I don’t know what it all means yet, but it’s intriguing.