Ethereum offers cheap data storage by allowing users to earn small quantities of money by renting out their own hard drives and unused space to drive down costs of file storage. The contract for this works such that you split the desired data into blocks, encrypting each block for privacy. You then make a contract with the rule that, ever x blocks, the contract will pick a random index and give y ether to the first entity to supply a transaction. So long as a contract is still paying out money, you have cryptographic proof that someone is still storing that block of your file.