Woleet.ID is an application provided by Woleet and available on mobile phones and on Ledger Nano S hardware wallets.
Woleet.ID allows to sign files using a bitcoin identity (key pair) you generate and own. Signing is as simple as scanning a QR code and generates timestamped proofs of signature (see https://medium.com/@woleet/beyond-data-anchoring-bee867d9be3a).
Signature request QR codes can be created using Woleet.ProofDesk (https://app.woleet.io).