EthNotary

This service provides a way to publicly verify signatures on a digital document without the need for a trusted outside party.
Using the Ethereum blockchain, we can store a hash of a private document (a contract, for example) along with an Ethereum Address. This proves in a public and secure way that the owner of the Ethereum Address has signed the document. Other parties to the contract can sign it as well. All they need is a copy of the original digital document.