phpListings.com - Start your own business directory website with customizable widget-based templates and effective listing monetization tools.
Keycloak - Open Source Identity and Access Management for modern Applications and Services.
Brilliant Directories - Brilliant Directories offers member management solution to streamline all aspects of membership website or association.
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
HivePress - HivePress is a free WordPress plugin that allows you to build any type of directory and listing websites. It's incredibly easy to use and highly customizable.
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.