"ZKCP is a transaction protocol that allows a buyer to purchase information from a seller using Bitcoin in a manner which is private, scalable, secure, and which doesn’t require trusting anyone: the expected information is transferred if and only if the payment is made. The buyer and seller do not need to trust each other or depend on arbitration by a third party."

One obvious, and sad use for this is Cryptolockers. I prove that I have the key to decrypt your files, you prove that you have the BTC to pay for it.