Ren, previously known as the Republic Protocol, is a decentralized dark pool exchange protocol; designed to support large volume trades, without effecting the prices of the assets being traded. The blockchain-based, dark pool protocol has built in privacy mechanisms and is provably fair. It uses secure multi-party computation (sMPC), and a network of Darknodes, to match orders without exposing their volume or price. Their private order book will also enable cross-chain trades of tokenized assets. The Singapore based company is also developing RenX, a decentralized dark pool exchange, built with the Republic Protocol. REN is the network's utility token. It provides security to the network, and is staked to enable the operation of a Darknode in the network.

0x is a decentralized exchange built on the Ethereum blockchain that allows for ERC20 tokens to be traded. 0x hopes to serve as a building block for creating decentralized exchanges.
It is an open-source system of smart contracts that acts as a platform for those wanting to build with the 0x protocol. Augur, Aragon and several other projects are using 0x, at the moment.
The 0x protocol addresses some of the common issues that plague other decentralized exchanges. 0x is faster, more liquid, and less expensive to run. Its speed is due to reduced blockchain congestion. This is achieved by moving all transactions off of the blockchain and then verifying them. This move, also allows for the elimination of transaction fees by reducing the reliance on the blockchain. 0x doesn’t charge fees of any type to use the protocol. However, Relayers that create their own decentralized exchange can charge fees.
The 0x Project uses the 0x Ethereum token, known as ZRX, that allows users to pay Relayers’ transaction fees. It is also a decentralized form of governance for the 0x protocol system. Anyone holding ZRX can give input about improving the protocol.