The Logitech PowerShell Controller + Battery ($99.99), for the iPhone 5, 5S, and fifth-gen iPod Touch, is a battery pack case that doubles as a game controller with its own set of physical buttons. Read the full review.
Snap an iDevice in, and your iPhone/iPod suddenly looks like a throwback handheld game.
The PowerShell Controller connects via Lightning; you just snap your iPhone or iPod in to connect. All the buttons are pressure-sensitive and analog, and there's also a dedicated pause button for games.
iOS 7 supports game controllers, and this is one of the first to take advantage of the MFi controller API. Hundreds of games are already theoretically compatible, but finding them can be a challenge. (Some good starting ones: Lego Lord of the Rings, Oceanhorn, Pac-Man, and Galaxy on Fire 2.)
A 1,500mAh battery lurks inside the PowerShell -- you can charge it up via Micro-USB and it recharges your device while plugged in.
The PowerShell has a D-pad, four front buttons, and two shoulder buttons, but it lacks extra buttons that are on the competing Moga Ace Power; it has no dual analog pads, and no extra triggers.
The fit and finish on the PowerShell seem good, and it feels a little more pocket-friendly when inserted than the Moga Ace Power.
You can still access volume controls while your device is popped in, too. You could even make calls, but that might turn heads.