Authentiq - Strong authentication, without the passwords
TypingDNA Authenticator - 2FA codes in your browser, secured by the way you type
Cotter - One-click phone number login and checkout
SuperTokens - Open Source User Authentication - An Alternative to Auth0 / Firebase Auth / AWS Cognito
Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.
Amazon Cognito - Amazon Cognito lets you add user sign-up, sign-in, and access control to your web and mobile apps quickly and easily. It scales to millions of users and supports sign-in with social identity providers and enterprise identity providers via SAML 2.0.