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.
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.