There have been a few game controllers released for iOS devices like the iPad and iPhone, but none that I've seen that work as elegantly as the 60beat GamePad promises it will. The GamePad offers all of the buttons you'd expect on a HD generation video game controller (with analog sticks and triggers and so on), but instead of using the iOS device's 30-pin connector, it hooks up through the headphone port, and apparently passes audio information across there somehow.
Unfortunately, because that's such an interesting connection, the controller only works with two games at the moment, Bugdom and Aftermath. But the company is promising more compatibility next year, and hopefully as more developers jump on board, we'll see more widespread support.
It's certainly an interesting prospect. The controller itself is $50, but as it works with more games, that price will definitely become more and more worth it. I personally have been using the OnLive wireless controller to play those games on my iPad (which still isn't available to the public, unfortunately), and I've been wishing for a controller like this, so hopefully 60beat will get the support it needs from devs.