The Western Digital World Edition is good for SOHO. Easy, but not too simple to be useful.
We don't actually use it, but our own version. For a geek, any left over old PC is a prime candidate to become NAS. The World's Edition internally runs Linux, so it's foundation is similar to the 20TB systems we use in the datacenter.
For a powerful and therefore somewhat complex system, have a look at OpenFiler. It's even more like a serious datacenter system, with a web based GUI. There is a learning curve to using a system that powerful and flexible, though, so the World Edition is a better fit for the typical Windows office.
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