Mucca is our latest favorite: a simple travel wallet named after Mucce & Buoi - a Milano Macelleria serving the most fantastic Steak. Mucca is made from a water- and tear-proof paper. Not even your washing machine will hurt it. Write your name on it in case you lose it, your phone number, mark the contained currency. But don't write your PIN on it.
This slimmest wallet of all comes in a pack of 3. It can hold up to 4 credit cards (in a separate slot on the back) and fits snugly around most currencies.