The iFrogz Caliber Advantage clips on to the rear of an iPhone, and provides slide-out extensions with traditional D-pad gaming buttons their faces.
By sliding out, these control surfaces offer a big advantage over other, fixed-size gaming peripherals. If you have this device attached to your phone, you can slide the gaming controls away, tucking them neatly behind the main screen, when you're not using them.
The resulting package is really not much thicker than your phone would be in any robust case, and some increased protective shielding of the phone's rear is provided. The slight increase in weight and bulk is, for many gamers, a worthwhile trade-off of the vastly increased mobile gaming functionality the peripheral provides.