Ethereum Classic is the original protocol of Ethereum. As a result of the massive hack on the Ethereum-based DAO, in which around 14% of all ETH in circulation were stolen, a hard fork was proposed to return the stolen funds to their owners. This caused an ideological split revolving around the question of changing previous transactions in the blockchain. Some Ethereum holders rejected the hard fork, and decided to keep using the original protocol, based on the principle that the blockchain is immutable, and cannot be changed.

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

Qtum has been described as a hybrid between Ethereum and Bitcoin. Qtum is a smart contracts platform that places a strong focus on value transfers as well as decentralized apps. Qtum enables users to create smart contracts on the platform and conduct cryptocurrency transactions.
Qtum has a Proof-of-Stake (PoS) consensus algorithm. Several decentralized apps are already using the platform.