FreeNAS is good, I ran one in various forms for years. My last one was an ITX atom based, with a WD green 2TB hard drive and an 80+ PSU, so very power efficient. I found it to be pretty much bullet proof as a NAS, my only issue was that the version of FreeNAS I used was a beta and I never quite got the plugin's working properly for streaming to ipad etc. so I would make sure that the current release has the plugin system working and test it, or go with an older more tried and tested version. There are plenty of tutorials out there as its quite a popular OS, and it will run on just about any old hardware you have knocking around. Plus, it's FREE!