"Being part of the Ayatana project, a keyboard shortcut hint
overlay will be displayed in the upcoming Ubuntu 12.04 LTS (Precise
Pangolin) operating system every time a user presses and holds the
Super key (also known as the Windows key).
"At the moment, in the supported versions of the Ubuntu
operating system that support Unity, pressing the Super key does
nothing but displaying numbers on top of the app shortcuts present
on the Unity Launcher. Pressing the number displayed on the
respective shortcut will rise up the window or start the app."