For years there've been a number of Usenet newsgroups for adventure games: rec.arts.int-fiction and rec.games.int-fiction for text adventures, comp.sys.ibm.pc.games.adventure for PC adventure games, and comp.sys.mac.games.adventure for Mac games.
Since the mid-1990's, Dejanews -- later called Deja.com -- provided quick and easy access to newsgroups. You could read groups and post new messages for free using Deja. They also archived all of Usenet, going back years and years.
All good things must come to an end, or at least decay into nigh-unusability. Deja's interface became more and more cluttered, with ad links placed around the messages, then finally embedded in the messages themselves. Finally Deja shut down, its archive of postings bought by Google.
Google has made most of Deja's archives available for searching, and you can read newsgroups and post to them from their page. But perhaps you're not fond of Google's interface, or would like something a little less cumbersome for reading and posting.
Even with Google's near-dominance, there are still some sites which offer web-based access to Usenet newsgroups, some even free. For example, Usenet-Replayer and Tera News offer free access. If none of those fit your needs, there are several for-pay sites such as Giganews Newsgroup Access.
Eventually, though, you may find that web-based newsreading doesn't offer quite what you need. In that case, you'll need two things: a newsreader and a newsfeed.
A newsreader (or newsgroup client) is a program which handles the details of organizing and presenting messages to you. A good client should have several features. It should allow you to view messages as threads, grouped by subject instead of by date; filter posts by subject and person who posts via a killfile; and read posts off-line, instead of forcing you to be connected to the Internet while you are reading posts.
There is a plethora of newsreaders for every platform. For Windows, the most commonly-used ones are Forte Free Agent, Xnews, Outlook Express, and Netscape. I'd recommend the first two over the latter, though if none of those are what you're looking for, there are plenty more. MacOS users can choose from YA-NewsWatcher, MacSOUP, Microsoft Entourage, and many more. If you're running Linux, your choices include trn, tin, Pine, and (my personal choice) gnus.
Once you have a newsreader, then what? You need a newsfeed, that's what. There is a chance that your ISP will provide you with a newsfeed. You'll need to ask them for the details. If not, there are a number of places you can get newsfeeds, known as NNTP servers. There are a lot of free ones available, but as in all things, you get what you pay for. Some free servers don't allow you to post; many vanish with no warning. The for-pay services mentioned above offer both web-based access and a newsfeed, allowing you to access newsgroups both through a client and through your browser.
There are a number of ways for you to read and post to newsgroups, as I hope this article has shown. And if you've never read any of the adventure game newsgroups, now's as fine a time as ever to start.