The 2 main providers are :
http://usenetserver.com
http://giganews.com
There are Back-end Usenet Providers (the ones with the real servers) and usenet-resellers (the one renting the servers to the former, so they don't have to manage the servers themselves, like your ISP)
back-end providers :
giganews.com
usenetserver.com (bought by highwinds)
newshosting.com (bought by highwinds)
easynews.com (bought by highwinds)
supernews.com
astraweb.com
nntpserver.com (closed)
newsfeeds.com (closed)
highwinds.com (not for end users, only for resellers)
readnews.com (not for end users, only for resellers)
etc.
usenet resellers :
ngroups.net (highwinds)
newsgroups.com
newsgroupdirect.com (highwinds)
powerusenet.com (giganews)
usenet-news.net (readnews)
newsdemon.com (highwinds)
just4today.net (very low retention, new login/password everyday, no SSL, but free access).
etc.
recommended :
giganews
usenetserver
astraweb
newsdemon
Usenet-News (seems nice with their pay-per-downloaded data, owned by readnews which seems a good provider).
(all data taken from
here).
About the newsreaders :
NewsLeecher once had a free account creation while registering a special account on giganews, but it's now terminated.
I don't know any other news downloader coming along a server.
If you want a free one, you can use
grabit.