Logitech Joystick for iPad ($19.99) - Logitech is going all out on tablet accessories. This joystick for the iPad attaches to your tablet using suction cups. It’s usually used on the left side for your thumb for games that use a directional pad. Logitech has a list of compatible games here.
Joystick-it iPad Arcade Stick ($24.99) - This tablet accessory goes retro with a Joystick you would normally see in an old school arcade. The arcade stick works similarly to the Logitech Joystick, but with a raised ball. The Joystick-it is good for older arcade or Atari type games. You can purchase two and use them on the right and left sides of your tablet for dual-stick gaming. Can be used on any capacitive tablet (not just for the iPad).
New Button Gaming Controls For iPhone 4 ($11.99) - These gaming buttons are made for the iPhone, but work just as well on the iPad. The nice thing about these is that they come with two types of buttons—one is a classic directional pad style, like you would see on an NES. The other is a diamond button configuration (like you would find on a Sony Playstation). Since these were intended for an iPhone, they’re considerably smaller than the other joysticks so they won’t take up as much screen space. RoboTouch - This is for the more adventurous type and requires some electronics know how. ProtoDojo created a hack that allows him to use an old NES controller to control his iPad. He uses conductive arms connected to micro servos that act as if his finger is actually touching the screen. The nice thing about this is that he can reposition the servos to the buttons on the screen. It’s a great hack and it would definitely be interesting to see if someone could put this out in the market.