Binary package hint: metacity
I can see behind the current window through a gap between the titlebar and the menubar in a wobbling window. Screenshot attached.
DistroRelease: Ubuntu 10.04
Package: metacity 1:2.30.1-0ubuntu1
ProcVersionSignature: Ubuntu 2.6.32-22.33-generic 18.104.22.168+drm33.2
Uname: Linux 2.6.32-22-generic i686
Date: Wed May 12 17:53:58 2010
InstallationMedia: Ubuntu 9.10 "Karmic Koala" - Release i386 (20091028.2)