Spring Security - The Spring portfolio has many projects, including Spring Framework, Spring IO Platform, Spring Cloud, Spring Boot, Spring Data, Spring Security...
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.
CAS - Apereo CAS - Enterprise Single Sign On for the Web
JSON Web Token - JSON Web Tokens are an open, industry standard RFC 7519 method for representing claims securely between two parties.
Firebase Authentication - Application and Data, Application Utilities, and User Management and Authentication