Keycloak - Open Source Identity and Access Management for modern Applications and Services.
Authelia - Authelia is an open-source authentication server providing SSO and two-factor for your applications running behind NGINX.
Shibboleth - standards based, open source software for web single sign-on across or within organizational...
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
Hiboo SSO - Single sign-on solution specifically built for modern FOSS hosted services.