Usenet servers (the free ones that provide access to text groups - which is what we're interested in) differ from one another. Some require registration to browse, read, and post messages, while others only require registration for posting. There are also some that allow all of these actions without needing an account.
Each server carries roughly 95% of the same text groups. What you post via server X will sooner or later appear on servers Y and Z, etc.
Below, you can find a list with most popular servers:
Also see pugleaf.net - dozens of servers in various locations, available via Tor and Yggdrasil network, web frontend access only (no posting yet).
There are hundreds, maybe even thousands, of other Usenet servers. On the page https://top1000.anthologeek.net you can find the “top1000” list of servers - those that inject the largest number of articles into the network.
Personally, I use and recommend paganini.bofh.team. It’s stable, has no downtime, doesn’t require registration, and is well-regarded by other users.
If you need to post a message of high importance and want to keep your identity relatively anonymous - to avoid being easily doxxed, for instance - consider using a remailer. More information about this can be found in posting anonymously ...
Some servers are anti-anonymous. They intentionally include the Injection-Info header in outgoing posts, revealing the posting user’s name and IP address. Before choosing a server for everyday use, it’s good practice to check what information it adds to your message headers. You can read how to test your server/client setup in this article.