Metamask is quickly becoming one of the most interesting projects to me. It's so easy to spin up a site now and use metamask for identity and authorization. If you have API's that need to be protected from denial of service attacks, you just channel them through a smart contract and make the user's pay the gas, no more need to protect your backends.

The blockchain just becomes a source of truth and a protection against DDoS attacks. Past that, all other "transactions" can be done off chain. Future is exciting!