Cotter - One-click phone number login and checkout
SuperTokens - Open Source User Authentication - An Alternative to Auth0 / Firebase Auth / AWS Cognito
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.
Authpack - Complete user auth system installed with 8 lines of code.
TypingDNA Authenticator - 2FA codes in your browser, secured by the way you type
MojoAuth - Allow users to login without passwords