0x allows users to trade ether using off-chain order books

First (formally) proposed through a whitepaper published on February 21st, 2017, the 0x (ZRX) protocol outlines the specifications for a decentralized cryptoasset exchange built on the Ethereum blockchain.
The whitepaper‘s abstract notes that the 0x (ZRX) protocol aims to “facilitate low friction peer-to-peer (P2P) exchange” of ERC-20 compliant tokens on the Ethereum network. Authored by Amir Bandeali (a University of Illinois Finance graduate and the CTO at 0x)