The Arduino Leonardo’s big new feature (thanks to its ATmega32u4 chip) is the ability to emulate a keyboard, a mouse, or a joystick. This brings up so many possibilities for alternative control schemes for a computer.
In this article they’ll show you how to grab the input from a Wii Nunchuck and get your computer to detect it as a mouse : http://www.forkrobotics.com/2012/06/leonardo-wiichuck-mouse/
In this one, they’ll show you how to got Arduino Leonardo appearing as a 4-axis 8 button joystick : http://forum.freetronics.com/viewtopic.php?t=734
Download Arduino Leonardo as a joystick lib : Leostick Buttons lib