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 protocol – Bitcoin 2.0 presentation – PHP and Python libraries.
blockexplorer blockstream blockchair