The app turns your iPhone or iPod touch into a wireless virtual controller with a configurable set of buttons. It syncs up to a desktop client and remaps the buttons on the Joypad to keyboard buttons on your desktop. It's very clever, and is a free download.
Taking the app to the next level, the two-man development team, Zell Interactive, offers several sets of skins to give Joypad that retro feel, including classic NES and Super Nintendo controllers. Sounds perfect for some MAME-emulation, doesn't it?
Finally, the makers of Joypad have developed an iOS SDK to allow other developers to incorporate Joypad support into their games. Similar to how the Scrabble iPad game lets users store their tiles on their iPhone to hide them from other players, JoyPad let's you control a game on your iPad from your iPhone. It works via Bluetooth and it's awesome:
The app itself is free, with 8 skins available individually for $0.99 or all together for a discounted price.