A nice project from user Batseke showed up recently on the web. It looks like a miniature gameboy with dimensions of a bussiness card. It is powered  by an Atmel TQFP32 which has enough juice to let you play tetris or flappy bird in its Oled display. Thickness is just 1.6mm and a coin cell battery will be enough for 9 hours playtime. Bateske is planning to launch a campaign in  kickstarter so if you interested keep an eye in this link: http://www.bateske.com/arduboy/