The accelerometer on the back of the pad.
A cunning electronics geek called Adam Thole has created a motion controlled Xbox 360 control pad, similar to the Wii and the PS3 pad.
Adam has taken an accelerometer circuit, disassembled the pad and mapped the circuit to the left analogue stick. The result is a nifty mod that works just as you'd expect - check out the video over on his blog
The initial version was built with a breadboard, but Adam tidied that up and ended up using complex surface mount components. The power comes through the USB cable, so the controller isn't wireless, but it's a pretty neat achievement nonetheless.
Check out the blog for the gory details, then let us know your thoughts on the mod over in the forums.