I use Keyoxide, developed by @yarmo.
• it’s fully open-source and can be self-hosted
• it relies on already existent OpenPGP identity proofs via notations — your identities never leave your OpenPGP key
• as such, one doesn’t need an account
• one can verify XMPP, IRC, Mastodon, Pleroma, Owncast, etc. New proofs are easy to implement
My favourite feature is the ability to use WKD to be able to use email addresses instead of fingerprints, like https://keyoxide.org/me@kytta.dev