I haven't noticed anybody mentioning this yet. It isn't useful at all, but here goes:
Open a Carbon or Cocoa app and dock one of its windows. Then hide the app (command-H or from the app menu).
The docked window "flies" over to hide behind the app's icon in the Dock! Unhide the app by clicking it in the Dock (or command-tab) and it flies back.
It's this kind of attention to detail that makes me love Apple. Now if only the interface wasn't so slow. :)
[Editor: Make sure your dock is visible first; hit CMD-OPT-D if you usually leave it hidden; it's not so impressive when you can't see it ;-) ]
Mac OS X Hints