OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.
Okta - Enterprise-grade identity management for all your apps, users & devices
Univention Corporate Server - Univention Corporate Server UCS with Active Directory functions and App Center is the cost-efficient and popular successor to Microsoft's server solutions.
Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.
FreeIPA - FreeIPA is an integrated security information management solution combining Linux (Fedora), 389...
OneLogin - On-demand SSO, directory integration, user provisioning and more