Guest pick from Jared:

The PocketTaco is a compact, foldable mobile gaming controller designed to mimic a Game Boy utilizing a mobile phone.
It is lightweight and fairly comfortable. I’ve used it only with the Delta emulator app. My experience is that it typically works better with the Game Boy and GBA emulators and not so much with other platform emulators. For only about $35, this is a neat portable gaming accessory.

Jared talked about this on episode 539: The iPhone has traveled further in space than any other phone