Turn almost any smartphone into a miniature gaming machine with the iControlpad. This gaming controller connects to your phone via bluetooth which means you can not only use it with your phone, you can use with tablets or computers and more. Shown above with an iPhone but it actually uses a clamp to connect to virtually any size smartphones- see the backside below:
If you use a slim case, you can probably use it with the phone still in the case. It has a built-in 1350mAh battery that not only powers the controller but it can also give your phone extra juice via USB. The controller has dual analog nubs, digital D-Pad, 6 face buttons, 2 rear buttons and is compatible with hundreds of official iPhone and Android apps (clickthrough for more details). Includes a firmware chip inside for future upgrades.