Recent Metadata in the Bitcoin Blockchain

Below is a list of metadata recently embedded in the bitcoin blockchain using OP_RETURN outputs.
This method was made official in Bitcoin 0.9 due to user demand. Opinions differ strongly and the future is open.

Also from Coin Sciences: MultiChain for private blockchains with full compatibility with Bitcoin Core.

More on OP_RETURN: CoinSpark protocolBitcoin 2.0 presentationPHP and Python libraries.

Transaction ID