Twelve South (http://twelvesouth.com) has introduced BackPack, a
shelf designed specifically to fit the iMac and Cinema Displays. As
the name suggests, the perforated metal shelf attaches to the back of
Macs and can be used to store hard drives, hubs, iPods and other Mac
peripherals.

It's available at Apple stores and online for US$29.95. The BackPack
uses two adjustable clips and gravity to literally perch on the desk
stand of iMacs and Cinema Displays. The adjustable clips allow users
to position the BackPack at the desired height, so gear on the shelf
is within reach.

Since it's height adjustable, there's room for two BackPacks on one
Mac. With two BackPacks in place, the lower shelf could be used to
store larger hard drives, while the top shelf could be used to store
smaller items like a charging iPod, or a favorite collectible.