I was looking at the page for the raspberry pi zero's product page and saw they used a 5 dollar bill for scale. It is easily smaller than the width of the bill, and that got me thinking, how hard would it be to put a pi zero in your wallet, with a fairly flat battery and miniature display. Extra points for a camera and any other sensors you can get in there. I would love to see an insert for a wallet the size of a few connected credit cards that could flip open like a tricorder.