You can try FreeNAS.
FreeNAS is a free and open-source software network-attached storage
(NAS) system based on FreeBSD and the ZFS file system. It is licensed
under the terms of the BSD License and runs on commodity 32 and 64-bit
x86 hardware. FreeNAS supports Windows, OS X and Unix clients and
various virtualization hosts such as XenServer and VMware using the
CIFS, AFP, NFS, iSCSI, SSH, rsync and FTP/TFTP protocols.
FreeNAS is managed through a comprehensive web interface that is
supplemented by a minimalistic shell console that handles essential
administrative functions. The web interface supports storage array
configuration, user management, sharing configuration and system
As an embedded system appliance, FreeNAS boots from a 2GB image that
typically resides on a USB Flash device. The FreeNAS operating system
is fully independent of its storage arrays, allowing its configuration
database and encryption keys to be backed up and restored to a fresh
installation of the OS. This separation also allows for FreeNAS system
upgrades to be performed though the web interface.
-- from Wikipedia
Important: The hardware requirements for the current version of FreeNAS (FreeNAS 8) can be a bit intensive ( up to 8GB depending on the filesytem used). The previous version is being currently developed under a different name (NAS4Free), and it can be used with even 192 MB of RAM!