Open-sourced tool from Distributed Lab - w3sign.app
https://w3sign.app
Link to the source:
https://github.com/dl-w3sign
W3sign is the simplest app that allows you to sign documents with Metamask and anchor them to the blockchain.
A couple of features:
Public mode. You can anchor the document and allow everyone to sign that. It looks like petitions, isn't it?
Permissioned mode. You can define the set of signers (their account addresses) who can sign the document and see who has already signed that.
W3sign uses zero-knowledge proofs for front-running attacks protection (nobody can steal the document hash and use it in advance)
Backendless. Nobody except counterparties stores the document. You can sign it with W3sign, send it through any channel you prefer, and let to verify only by a permitted party.
Deployed on Ethereum, Q, and Polygon blockchains. Connection to the new EVM network can take ~30 minutes.