NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
ClearOS - A very good alternative for Windows Server, for free
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.
Ubuntu Server - Secure, fast and economically scalable, Ubuntu helps you make the most of your infrastructure. Whether you want to deploy a cloud or a web farm, Ubuntu Server supports the most popular hardware and software.
FreeIPA - FreeIPA is an integrated security information management solution combining Linux (Fedora), 389...
OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.