CryptoProof
CryptoProof ran from 2018 to 2021, as a free timestamping service on Ethereum, built by jonas.eth. As transaction fees picked up, running CryptoProof as initially, naively designed became unsustainable.The idea is now on hold. It likely will be revived as zk rollups become available. Feel free to contact me for any inquiries about CryptoProof.