What is the "show desktop" keyboard shortcut?

For 11.10 and 12.10

Control-Alt-D shows the desktop for me

For 12.04 and up

Control-Super-D works for me

• What are Unity's keyboard and mouse shortcuts?

In Settings, select keyboard then go to shortcut tab -> navigation -> Hide all normal windows -> click on predefined shortcut and press your preferred combination of keys such as Super and D.