Cotter - One-click phone number login and checkout
Feather Auth - Build an entire sign-in flow in 5 minutes or less.
SuperTokens - Open Source User Authentication - An Alternative to Auth0 / Firebase Auth / AWS Cognito
Authpack - Complete user auth system installed with 8 lines of code.
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.
Authy - Best rated Two-Factor Authentication smartphone app for consumers, simplest 2fa Rest API for developers and a strong authentication platform for the enterprise.