OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
Rockstor - Rockstor is a free and open source NAS (Network Attached Storage) operating system.
ClearOS - A very good alternative for Windows Server, for free
Amahi Home Server - Amahi is a media, home and app server software known for its easy-to-use user interface. Amahi has the best media, backup and web apps for small networks.
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.