Put the display to sleep without putting the whole computer to sleep?
You can turn off your iMac (and your macbook) screen without sleeping via the following keyboard shortcut:

 ⌃  +  ⇧  +  ⏏ 

(Control +SHIFT + EJECT)

Machines like the Air and newer MacBook Pro with no optical drives have the power button in the same place as the eject key and it works similarly for display sleeping shortcuts.