A fellow who goes by the name [c.invent] designed and built this open-source keychain-sized multi-platform emulation console, called the Keymu. It uses an Intel Edison (a computer-on-module). a 1.5-inch OLED display, and 11.7mm speaker, and a 220 mAh lithium battery, all inside a 3D printed clamshell case. You can learn how to make your own at Hackaday.
