Univention Corporate Server (UCS) is a free platform designed to operate and manage server applications, user identities, and complete IT infrastructures. Whether you are aiming to deploy Microsoft Windows, Mac OS X or Linux systems: UCS is suited perfectly for the management of distributed, heterogeneous and virtualized IT environments. Its integrated identity management system allows the simple administration of users, identities, applications. Firthermore, the central management of server and desktop systems can be managed via an intuitive web interface. UCS has its own App Center where users find multiple applications (like Nextcloud, ownCloud, Collabora, ONLYOFFICE, Jitsi and many others). These apps can easily be integrated into the IDM of UCS. Users can easily access these applications via an online portal via single sign-on. Another prominent feature is that UCS offers AD functionality and an AD Connector to operate UCS parallel to a Windows Server.
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
TrueNAS Core - TrueNAS Core (formerly FreeNAS) is a storage operating system strong and robust enough to meet the needs of enterprise level businesses.
NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.
ClearOS - A very good alternative for Windows Server, for free
XigmaNAS - File Sharing, OS & Utilities, and Security & Privacy