RFIDs are showing up in everything from your cards to your passports, offering easy identification but also a greater chance of identity theft. Although plenty of RFID-blocking wallets exist, none are quite as thin and pocketable as the HuMn.
The HuMn wallet is essentially a plate with a band to keep your cash and cards in a small space, but in addition to its minimal carrying functions it also blocks any RFID skimming. They start you off with two plates, but you can add on more if you want at any time.
Currently the HuMn wallet is available on a more-than-funded Kickstarter project that's basically now a pre-order page. You'll probably have to wait a little while to get yours, but a modest investment will get you one of these thin and protective cash carriers as soon as they're ready.
UPDATE: Apparently there's already a wallet just like this, and it's called the William Wallet made by Wintercheck Factory. If you're looking for an option you can buy now (and seems to have thought of this idea first), check them out.