A while back (I missed it at the time) Arstechnica had a "shootout" between two of the free NAS distributions - FreeNAS and NAS4Free. Both are (as stated) free and ready to run the ZFS advanced file system for rock solid data reliability.
FreeNAS is a rewrite of the original FreeNAS code, and NAS4Free is a derivative from the original code. So, both had the same origin, but have moved in radically different directions. Both are full featured NAS distributions capable of being run from a thumb drive.
The fundamental differences are discussed in the article, but a summary of the interface differences states:
FreeNAS also tends to be noticeably bleeding-edge, which may delight you as a technical enthusiast or quietly horrify you as a more conservative sort of techie. By contrast, NAS4Free dispenses with the fancy stuff and sticks with a more down-to-earth and focused "I am a NAS and I am good at it" game plan. You don't get newsgroup readers or bittorrent clients or owncloud servers, and you can't make your own domain controller with it, but NAS4Free has a much cleaner, more usable interface. It readily exposes more of the information you actually need without cluttering things up with glitz that you don't. In my opinion, NAS4Free's Web interface tends to just feel more solid and reliable.
The walkthrough of both NAS distributions is quite complete with many screen shots. If you are considering using either, this multi-page article is highly recommended.
- NAS4Free vs FreeNAS vs Amahi vs OpenMediaVault (homenasreviews.com)