No, it is not free ($49), but if you are serious about Newsgroups, you will not do better than Hogwasher. I have used about all the newsreaders available for the Mac and found them all lacking in one area or another, but not Hogwasher. It is quick, accurate, and it downloads only what you tell it to, unlike many newsreaders that try to download all of a group of files if the subject of a posting is typed a certain way. It is worth every dollar if you download lots of files from Usenet. You can try it free for 60 days. After you use it, you won't want to use other readers I bet.
I must vote for thunderbird, not the best looking, but the price is right and it definitly gets the job done. It may lack some of the advanced features - but it's done everything I've needed thus far.
Another one I just found, but haven't tested is MT NewsWatcher also free.
Check out tucows they have a nice list.