Keycloak

Keycloak (sponsored by Red Hat) is a fiercely pragmatic, deeply engineered open-source leviathan that mathematically attacked the "SaaS Vendor Lock-In" problem. Auth0 and Okta charge massive premiums; Keycloak engineered a free, hyper-scalable Java backend. It is the absolute weapon of choice for hardcore enterprise Kubernetes teams who mathematically demand absolute sovereignty over their SSO engine, hosting it themselves to avoid paying $50,000 a year to a cloud vendor.