Here is a very interesting paper I read some time ago that analyzes the usage of Bitcoin OP_RETURN ("arbitrary data") transactions:
An analysis of Bitcoin OP_RETURN metadataOn page 6 they list some of the use cases:
Assets gathers protocols that exploit the immutability of the blockchain to certify ownership, exchange, and eventually the value of real-world assets. Metadata in transactions are used to specify e.g. the value of the asset, the amount of the asset transferred, the new owner,etc..
Document notary includes protocols for certifying the ownership and timestamp of a document. A user can publish the hash of a document in a transaction, and in this way he can prove its existence and integrity. Similarly, signatures can be used to certify ownership.
Digital arts includes protocols for declaring access right and copy rights on digital arts files, like e.g. photos or music.
Other includes protocols whose goals differ from the ones above. For instance, Eternity Wall [20] allows users to store short text messages on the blockchain; Blockstore[13] is a generic key-value store, on top of which more complex protocols can be implemented
Some examples: Asset protocols: OpenAssets, Omni or Counterparty, Document notary protocols: Factom and Proof of Existence, Digital Arts: Monegraph and Blockai, and so on.
