Keycloak
Keycloak is an open-source identity and access management solution that simplifies authentication and authorization for applications and services with minimal effort.
Keycloak is an open-source alternative to:

Keycloak is an open-source identity and access management solution designed to add authentication and authorization to applications and secure services with minimal effort. It eliminates the need for applications to handle user storage, authentication, or login forms, providing centralized management for both administrators and users.
Key Features
- Single Sign-On (SSO): Users authenticate once and access multiple applications without re-logging in.
- Identity Brokering & Social Login: Easily integrate social networks (e.g., Google, Facebook) or existing OpenID Connect/SAML 2.0 Identity Providers.
- User Federation: Connect to existing LDAP or Active Directory servers or implement custom user stores.
- Admin Console: Centrally manage applications, services, authorization policies, and user permissions.
- Account Management Console: Users can update profiles, change passwords, set up two-factor authentication, and manage sessions.
- Standard Protocols: Supports OpenID Connect, OAuth 2.0, and SAML 2.0 for broad compatibility.
- Fine-Grained Authorization: Define custom authorization policies beyond role-based access control.
- Themes & Extensibility: Customize the look and feel or extend functionality through code.
- High Performance & Scalability: Lightweight, fast, and supports clustering for high availability.
- Password Policies: Enforce custom password policies for enhanced security.
Tech / Self-Hosting
Keycloak is a Cloud Native Computing Foundation (CNCF) incubation project. It is open-source, self-hostable, and supports various deployment options including Docker, Kubernetes, and traditional servers. The project is actively maintained and widely adopted for enterprise and open-source use cases.