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.
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
JSON Web Token - JSON Web Tokens are an open, industry standard RFC 7519 method for representing claims securely between two parties.
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.