Apps similar to Microsoft Entra External ID include Auth0, which has a free version. Other options are Amazon Cognito, Google Cloud Identity Platform, Passage Identity and Azure Active Directory B2C. In total there are five Identity Management Tools like Microsoft Entra External ID for the web.
You can connect any application (written in any language or on any stack) to Auth0 and define its Connection, the method used to authenticate the users of that application:
United States
Amazon Cognito lets you add user sign-up/sign-in and access control to your web and mobile apps quickly and easily. Cognito scales to millions of users, and supports sign-in with social identity providers such as FB, Google, and Amazon, and SAML 2.0.
United States
Identity Platform is a customer identity and access management (CIAM) system that can help you add identity and access management functionality to your applications, protect user accounts, and scale with confidence.
Canada
Azure Active Directory B2C (Azure AD B2C) is a customer identity access management (CIAM) solution that enables you to sign up and sign in your customers into your apps and APIs.
United States
