Thunderbird is a good cross-platform solution, though its competences are basic by the standards of people who love Usenet. But it's easy to get for Mac, Windows, Linux, FreeBSD, etc.
On the Mac, though you have to pay for it, there's Unison, which is nice. Otherwise you can google MT-Newsreader or Pineapple Newsreader, both of which have been updated from the old Mac OS9 days and are useful and good software.
There are a ton of newsreaders for Windows. A lot of people like Gravity or Forte, but I'm not on Windows so don't use either. At work, where I am not allowed to install software on my Windows box I use Sylpheed from a pendrive, and you can get it at pendriveapps.com
Finally, Usenet on a smartphone or tablet is awesome! If you have an Android phone, check out Phonews or NNTP Newsreader, both of which have free and paid versions. It's the number one thing I do with my smartphone. And on the Ipad there's Newstap, which provides a better UI but which I find a bit slow if you have a long list of groups that you read.
See you on comp.misc at Usenet.
If you need a newsgroup/usenet provider I recommend newsgroupreviews.com. It's a site that takes advertising but it's run by guys who love Usenet. I personally pay for a provider, as it's better quality. I got a great deal at Blocknews. Individual.net is better quality because they keep it spam-free, but they don't carry all the groups, and comp.misc is one they don't carry. There are free providers out there, but I find they are often choked with spam and occasionally drop posts. That was the case for AIOE a while ago. Albasani.net is better, and eternal-september is also good.