One of the great things about the whole old-school gaming scene is the volume of supplement that are available. It really doesn’t matter which set of rules you may be using there’s plenty of “generic” supplements to help you out. Plus some of these are totally system neutral that you could use no matter what game you happen to be using.
Let me start with one that’s original. That is the best damned City supplement you’ll find and it doesn’t come from the modern era. That’s the 3rd Edition City Supplement from Chaosium.
Far as I know this isn’t available any more. So you’ll have to do some searching if you want to drum up a copy. It’s got random city encounters, building cities, and the infamous “Catch Up” tables. That’s for when the player is out but still wants some XP and stuff. Roll on the charts to see what you were doing. Of course, your character could die but that’s a risk you take. This book is standard in my GM bag when I’m running game.
The Tome of Adventure Design: This is pretty damned big book and isn’t meant to be used at the table. This is filled with inspirational charts and tables to design dungeons, villains and all sorts of things to throw into your campaign. Heck, just sitting back and reading it will start the ideas flowing.
Engineer Dungeons: This one is another aid with tables for creating dungeons. Like the Tome of Adventure design, you may not want to use it at the table but is filled with inspirational material. Sitting down with these two will let you design an adventure in an afternoon without lots of headaches.
New Big Dragon Games Unlimited Supplements: All of their stuff is pretty damned good but there’s three of them are a must haves. The d30 Sandbox Companion, d30 DM Companion, and the Creature Codex. These are great to have the table. The d30 Companions give you a wide range of random tables at your finger tips and who can’t use even more monsters to throw at the party.
Yes, I know that there are many, many more out there. Heck, I have even more on bookshelf. But I wanted to showcase what I consider the best of the best and those books that I end up using most often.