On some local networks (some university mainframe accounts), or text-only dialup (e.g. our community net uses a Lynx web browser for our text-only customers), one could access a few gazillion "newsgroups" like alt.fan.startrek (ad infinitum) by using the TIN newsreader.
You can also access newsgroups through the PINE email program, which is how I do it here in Halifax.
GUI newsreaders are out there a-plenty, though I think only those of us who grew up on the "old style" bother to look them up anymore... the new kids seem more drawn to Yahoo! Groups.
Stop over to http://www.versiontracker.com and plug in "newsreader" for a search - you'll find lots, with user ratings to weed out the not-so-greats.
Thoth is a program for OSX that allows you to look at threads. It's a bit hardcore and has many options that the newsreading novice might find confusing, but it works well. Here's a link to the developer...Thoththoth web site