I've released an ethereum smart contract + a client app for creating and verifying trusted timestamp on the ethereum blockchain. The service works in a similar way than BTProof  but using a generic/turing-complete blockchain makes it more efficient and cheaper for the user. Basically the idea is to calculate the `sha256sum` of a given file and then submit a transaction to the contract containing this hash. The contract goal is to associate the hash to the block timestamp, which is literally three lines of code .
With the latest C++ ethereum client (the `develop` branch) open http://ethereum-chronos.meteor.com
in the embedded web browser. Then you can simply drag and drop a local file. If a *proof of existence* exists in the blockchain for this file, the application will show you the associated date. Otherwise, a single click button will allow you to create such proof. As the contract, the app is open source and available on github .
I have attached some screenshots of the application, feel free to share your thoughts!