Uniswap is a decentralised exchange in the form of two smart contracts hosted on the Ethereum blockchain, as well as a public, open source front-end client. It's a 100% on-chain market maker allowing the swapping of ERC20 tokens, as well as ETH to an ERC20, and vice-versa.
Total Circulation: 1,000,000,000
Official website: https://uniswap.org/
Source code: https://github.com/Uniswap