Compound

Compound is a decentralized DeFi protocol for interest rate markets. When users supply ETH to earn interest, they "exchange" it for cETH, which they can exchange back for ETH at any time. This is the case for every token Compound supports. Compound does not touch fiat, derivatives, futures, options, etc. Users can simply supply assets; and borrow assets. This generates interest payments from borrowers to suppliers.

The protocol is free to use and openly accessible.