This seems to be the case for the Dungeon Crawl Classics rpg, with Metal Gods of Ur-Hadad and Crawling Under the Broken Moon leading the way. Metal Gods fleshes out the great city of Ur-Hadad, with info on it's gods and a street urchin character funnel. Crawling is a post apocalyptic zine set in Murica and each issue so far (issue two just came out) has a new character class, a weapons article, new creatures and new location/encounter. These two are fairly new to the scene, thankfully I caught an interview with one of the Metal Gods creators on Spellburn (I think this is the one) and I have a copy of CUBM #2.
Unfortunately I know less about Crawljammer, a pulp sci fi zine (Bat Men of Venus) with new classes and creatures. Along with Crawl! we seem to have the bases covered; fantasy, science fiction, and science fantasy. Since Goodman Games has been concentrating on releasing adventures for DCC, 'zines have been where you can get new additions for your campaign, and from what I have seen and heard, the quality is fantastic. From new and amazing artists to great and groovy writers, you can find them here. It is kind of like a new generation for rpg. I realize there was stuff like Alarums and Excursions back in the day, but unless you were in an active gaming community you would never know about it. The internet and podcasts allow creators and fans to talk about these things and "boost the signal" when it comes to these 'zines.
So, what if we had a 'zine that was almost like the old gazeteers? Maybe something giving flavor to different creatures, regional goblin tribes? Perhaps the Festering Slit tribe is fond of using poison? The Copperclaws might be open to negotiation. This can be done with so many things in so many ways it is really exciting. That is all I have time for now, I'll get back to this later.