Before the World Wide Web, USENET was the largest single source of
Internet activity. Even now it makes up a good portion of the overall
traffic.
It's a place where you can find discussions on pretty much every
topic imaginable. Unfortunately, it's also become a huge source of
unwanted advertising.
Below are some links to help get you started, but the best thing to do is just follow some newsgroups that you're interested in for a week or two and then jump in.