As Joshua mentioned, freenas.org is a good resource. You also might want to check out Lime Tech's unRaid as well. http://lime-technology.com/

I'm a masochist so I rolled my own, but there are better options out there if you don't want to tinker extensively. And tinker I did. Three full array rebuilds, at about 12 hours each, due to my inability to quit messing with things.