I recently acquired a Logitech PowerShell game controller, one of the first to support the new iOS 7 Game Controller API. It is a so called standard form-fitting controller, which is an enclosure for iPhone or iPod touch with a button layout similar to the trusty old SNES controller. As this should be a good match for some retro gaming, I searched the internets for suitable open-source emulators. You will need an Apple developer account to install any of those. This post will be updated with additional emulators as I go along.
There's a fairly new emulator called SiOS, which already has a fork with working controller support.
$ git clone https://github.com/GaryJohnson/SiOS.git -b controller_support
I opted for Nestopia here, as it already supports controllers out-of-the-box.
$ git clone https://github.com/jzdziarski/nestopia.git
Gearboy is a nice GameBoy emulator, but it lacked controller support. There's a fork by me which adds it and already gets rid of the GB chrome to make it work in landscape-right mode which is required for use with the PowerShell.
$ git clone https://github.com/neonichu/gearboy.git