One of my nephews is an incredible rhythm-game player. It's truly a sight to watch him smash buttons in what appears to be a chaotic, nonsensical fashion - only to look up at the screen and see him setting high scores.
For Christmas I made him a USB controller for one of his favorire rhythm-games, Pop'n Music. I designed this controller in Adobe Illustrator to emulate the official Pop'n Music arcade cabinet control board.
The base box was constructed by hand, but the top layers were milled on a CNC machine. The internal wiring is pretty basic, in that it is essentially a glorified USB joystick. Technically this could be used to play any game on any device that accepts USB contollers and allows for button mapping, as it is only switches connected to USB encoders.