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.
ClearOS - A very good alternative for Windows Server, for free
OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.
SME Server - SME Server is a leading distribution for small and medium enterprises.
XigmaNAS - File Sharing, OS & Utilities, and Security & Privacy