As a website owner, you would just register your site with a ring and then add a bit of HTML at the bottom of your page which would display the randomly-selected banner of another site in the ring (and your banner would sometimes be displayed on another site). So visitors could just click on the banners and be taken through a circuit of interesting (sometimes) websites.
In my experience a lot of programming blogs still have them, but then again those are the people who are most capable of adding them and who are mostly likely to find them