Authy - Best rated Two-Factor Authentication smartphone app for consumers, simplest 2fa Rest API for developers and a strong authentication platform for the enterprise.

Keycloak - Open Source Identity and Access Management for modern Applications and Services.

Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.

Clerk Authentication - Clerk is a complete suite of embeddable UIs, flexible APIs, and admin dashboards to authenticate and manage your users.

Google Authenticator - Google Authenticator is a multifactor app for mobile devices.

SuperTokens - Open Source User Authentication - An Alternative to Auth0 / Firebase Auth / AWS Cognito