decent free newsgroups dont really exist because it costs a lot of money to run it. However, some ISP do provide free newsgroup access. But, dont expect much. You can check if you ISP offers it here: http://docs.newzbin.com/Usenet:ISP_Providers
Also, every so often a "premium" server will over free beta testing. One that is currently doing that is easynews.nl. The retention is only about a week or two.
Server: news.easynews.nl
Username:
[email protected]
Password: beta
Honestly, ur best bet is to go get a paid server. UsenetServer is $13/month so its not too bad. That, and the retention is 90 days. ;)
EDIT: If you dont download too much, you can get pay per gig instead of pay per month.
http://news.astraweb.com/ and http://www.usenet-news.net/ are basically the only two usenet servers that still offer pay per gig.