The company behind NexentaOS is Nexenta. They use the OS to build a NAS appliance, NexentaStor, that runs on commodity x86/64 hardware, and offers very cool features: unlimited snapshots, snapshot replication, NFS, CIFS, FTP, Rsync and Amanda. Also, they use ZFS so you can grow your storage to extremely large pools, with both direct-attach storage or using iSCSI.
The cool part, the one that got me interested, is that they also make available a VMWare image, a NexentaStor Virtual Appliance, fully featured time-unlimited evaluation product (good for up to 6Tb) that you can download for free. I'll download the Quick Start Guide to see what can be done with this, but if this is really simple to use, it might just replace the small office NAS (around 500Gb RAID-1).
I found out about Nexenta via the Joe Little blog - Little Notes. Joe works at the Electrical Engineering department at Stanford University, and you can read how they are using Nexenta as a second tier storage for backups, keeping 6 months of daily snapshots. He also has a white-paper about multi-tier storage worth a read.
One last item: in the Community tab of the Nexenta website, I also found a RsyncShare, a open source implementation of RSYNC shares manager for Windows servers. It might be worth a look if you have to manage stuff like that.
I'll leave you with an article by Paul Murphy about OpenSolaris, NexentaOS and what they mean to Linux. Also very interesting stuff. The road to ZFS on Linux is still buried deep in incompatible licensing mud, so I don't expect to see a decent implementation running native on Linux soon. And I don't consider FUSE to be a production environment, sorry. So if running OpenSolaris or NexentaOS makes you dizzy, you might want to take a look at FreeBSD, given that ZFS support is already in there.