I don't know if wrapping your cash in a wallet designed to look like hundred dollar bills is the safest way to carry money, but the Tyvek Dinero Wallet ($15) is definitely the most durable. This wallet is made from a solid sheet of DuPont Tyvek®. According to this website, What is Tyvek?, it is a "material that offers all the best characteristics of paper, film and fabric in one material and that unique balance of properties cannot be found in any other material". Interesting stuff.