FreeNAS - compared to NAS4free the system requirements are slightly higher but does offer ZFS and other powerful features. I ran this off a 4gig thumb drive and didn't have to waste a drive bay on a system hdd.
NAS4free - worked better for me and allowed me to experiment with different formats like JFS for mythtv. I do not believe this offered ZFS but I could be wrong. I used a hardware managed RAID 5 array via IDE PCIX card and a software controlled RAID 5 via SATA PCIX card. -- hardware controller limited array size, software controller got around that.
OpenMediaVault - IMO the best user interface. I was also able to have the NAS watching my UPS via RS232 and other standard features like transmission.