The Makey Makey is a simple device that uses an Atmega microcontroller impersonating a USB Keyboard. I’ve built a version that uses an Arduino Pro Micro I call the Micro Makey-Makey.
The Micro Makey-Makey PCB board has connections for six connectors and two ground cables. This gives you enough to make a standard game controller.
Here’s a quick video on the Micro Makey Makey:
Here are some instructions and source code: