Hi qwertyjjj,
The question is a bit outside of the scope of FreeNAS, and the question doesn't exactly make sense, but if you are asking what the best free Usenet service is the short answer is there isn't any free service that's going to be worth using with SAB.
There are plenty of free Usenet providers out there and most of them will give you unlimited access to all the text-based groups you can think of, but none of them really provide access to the particular newsgroups you are (probably) going to want to use with a binary Usenet download tool like SAB.
Pretty much you are going to have to buck up and spend a few bucks for a proper Usenet account at one of the fine purveyors of NNTP access out there.
-Will