StableBit DrivePool - State of the art, file based drive pooling software.
ClearOS - A very good alternative for Windows Server, for free
Greyhole - Greyhole is an application that uses Samba to create a storage pool of all your available hard drives, and allows you to create redundant copies of the files you store, in order to prevent data loss when part of your hardware fails.
NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
btrfs - Btrfs is a modern copy on write (CoW) filesystem for Linux aimed at implementing advanced features...
FreeIPA - FreeIPA is an integrated security information management solution combining Linux (Fedora), 389...