Uniswap

Uniswap is the largest decentralized exchange by total value locked. It works as an automated market maker: instead of an order book, the price comes from a formula and the ratio of tokens in the pool. Version three added concentrated liquidity, letting a provider pick the price range where their funds work and earn more on the same capital. Version four opened pools to custom logic through hooks.

The protocol runs on nine networks, among them Ethereum, Arbitrum, Base, Polygon and Optimism. That makes it a convenient entry point for swapping almost any ERC-20 token, though on Ethereum the network fee for a swap can exceed the gain from the trade during busy periods.