Primecoin is a cryptocurrency based on finding prime numbers. Identifying prime numbers plays a central role in cryptography. Primecoin's unique Proof-of-Work (PoW) consensus mechanism is able to increase transaction speeds and improve energy efficiency.
The network searches for special prime number chains known as Cunningham chains and bi-twin chains, which are of interest in mathematical research.

I/O Coin is a Proof of Stake blockchain platform that aims to provide a variety of decentralized services. Including: payments, encrypted document storage, messaging, and offline transfers. The network is powered by the IOC coin.

OKCash is a Proof of Stake cryptocurrency that was released under the MIT licence.