The Rinkeby API is used for browsing non-fungible assets on the Ethereum Rinkeby test network. Creating and purchasing assets on this network requires Rinkeby ether, which is free. To acquire some, visit https://faucet.rinkeby.io/ and authenticate using your Twitter account.
The API is identical to the OpenSea mainnet API, except the base URL is https://testnets-api.opensea.io/api/v1.
This API is rate-limited. If you'd like to use it in a production environment, request an API key here.