To name a few off the top of my head: Google, GitHub, Gitlab, Facebook, 1Password, etc.
It would be nice if someone made a library that made incorporating Webauthn login into an app as simple as using django or Ruby on Rails or React to create a login form, so folks don't end up rolling their own and assuming that a user will have at most one yubikey.
Failing that, you could do what Zeit does and rely on email providers' support for Security Keys (login by email link only).