ClearOS - A very good alternative for Windows Server, for free
Webmin - Webmin is a web-based interface for system administration for Unix.
NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
FreeIPA - FreeIPA is an integrated security information management solution combining Linux (Fedora), 389...
cPanel - With its first-class support and rich feature set, cPanel & WHM has been the web hosting industry's most reliable, intuitive control panel since 1997.
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.