Gary Sims wrote an excellent and in-depth review of FreeNAS (a FreeBSD-based network attached storage system). His article details his experiences with FreeNAS, how it worked, where it (or he) failed, and other tips and tricks that he found as he went.
While FreeNAS appears to be the most popular (at least according to Google!) there are others out there, including OpenFiler (which is Linux-based).
A NAS basically is a dedicated file server that provides many different protocols to the clients and acts as an appliance. In some ways, this is no different than the historical file server – but in these cases, the NAS device is much more a turnkey solution with no other purpose. Many NAS systems support Windows file sharing, Macintosh file sharing, NFS, and a plethora of other protocols – all in order to make files available as much as possible. OpenFiler is one of these.
NAS devices were traditionally contrasted against SANs (storage area networks). The NAS provided a filesystem on the network; the SAN provides a block device on the network. This apparent sharp division of purposes does not exist in reality: some NAS systems also provide SAN resources as well.
Which – FreeNAS or OpenFiler – would I use? Can’t say – OpenFiler caught my eye first, but FreeNAS has the FreeBSD base. I’m liable to try both of them one of these days.
UNIX Administratosphere 
I’ve played with FreeNAS… Very cool, from what I could tell. Support for iSCSI, RSYNC over SSH (both client and server), Samba, NFS, etc… Very slick.
Let me know if you try OpenFiler.
Tom
tried out openfiler last night. I’d give it a 6 out of 10. seems to be principally aimed at folks who need a windows server. Has all the other protocols, but unless I’m missing something, the support for NFS is not great. I had to go to the cmd line to export shares for example.
Great article. Could you shine the light on a solving the following:
Many Pc’s, Many Macs, some unix.
One file server.
Done are the days of even using a ‘simpletech’ device (running a unix based app) to share between all.
I shifting from a NAS device to setting up a nas file server.
I’m not sure I understand, but FreeNAS should support all of those I do believe. UNIX generally supports NFS out of the box, and Macintoshes have supported Appletalk throughout (though NFS support has been “in the box” since MacOS X came out).
Windows supports Windows file sharing (called CIFS or SMB).
All of these should be supported in FreeNAS (that is, AppleTalk, CIFS, and NFS).