I've been very happy with OMV, for the short time I've been playing with it. Its FOSS and the web interface makes it very clear all the layers of abstraction you can use to manage a NAS. I highly recommend it.
And proxmox is good too, also FOSS (proxmox VE). I also has another slick web interface to manage stuff. I like the web interfaces because, albiet intimidating, it exposes alot of options available to me, which give me opportunities to research and understand how it works.
But I'm still working on getting everything with it set up, so take my suggestion with a grain of salt!