Twelve South, a Mac-only accessory design house located in Charleston, South Carolina, has just introduced the BackPack (US$29.99 or US$149.99 for six), a small adjustable shelf that uses friction and gravity to stay in place on the aluminum arm that holds up the body of the iMac and Cinema Display. BackPacks adjust to the width of the arm, so you can install two of them if desired.
While most Mac users will probably opt for hiding a hard disk, Twelve South shows BackPacks being used to display action figures and hold a flower vase (water in the vase is not recommended). My personal favorite in the BackPack gallery is a Mac mini perched behind a Cinema Display.
BackPack is a clever and well-designed product, and is available either at Apple Stores or through the Twelve South website.