Chainlink is the most widely-known Decentralized Oracle Network.
To be precise, Chainlink is a framework for DONs, and is a way to get data in from multiple sources across multiple oracles. In this context, Chainlink is not a data provider. It’s simply a decentralized network of centralized oracle node operators.
The Chainlink network uses a system called Off-Chain Reporting to reach a consensus on data off-chain, and report the data in a cryptographically proven single transaction back on-chain for users to digest.
Chainlink is entirely blockchain agnostic.