Wow, you wait for an Arduino Game Boy for years (without actually knowing that you're waiting) then two come along on the same day. Check out this beautiful and tiny design:
At 1.6mm thick and the footprint of a credit card this is some really neat work. It has capacitive buttons, a low profile piezo for sound and a tiny OLED screen, all running from an ATMega328. This doesn't have the depth in development of the other board featured today (see tab right) but it's a really nice concept none the less.
Again the creator wants to kickstart the design then Open Source it, potentially missing out on a lot of help with the development. Another design to watch. Check out the build log and website at bateske.com and the video demo here