Keycloak
keycloak.org
Open-source identity and access management — single sign-on, social login, and user federation as a self-hostable alternative to Auth0.
Why it is useful
Keycloak handles the full scope of enterprise authentication (SSO, LDAP/AD integration, social login, fine-grained authorization) that Auth0 charges per monthly active user for, with no user-count-based billing once self-hosted. It's backed by Red Hat and widely used in production, so it's a mature choice rather than an experimental alternative.
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