ZoneMonitor, Mystery Tool
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.