My review of Dungeon Delve got me to thinking about other products that have been useful to me in running games over the years - not planning a campaign, not organizing material, but actually running a game, with players. So I will talk about a few of them below:
For 1st edition AD&D it was definitely Dragon. Adventures, new monsters, new magic items, new NPC's, spellbooks, articles on and descriptions of monsters that lead to an idea on doing something new - old-school Dragon was a regular source of great fluff, great crunch, and a ton of usable stuff. It seemed to drop off to me somewhere in the early 100's but up until then there was something in almost any issue that I could use very quickly. I initially thought "Pages from the Mages"was a stupid series until my PC's killed a fairly tough wizard and I had no notes on his spellbook contents - guess what they found? I had PC's wash up on a strange island and I didn't want to use a "regular" monster - thank you Creature Catalog. Running into something you don't recognize after perusing the Monster Manual regularly for several years packs a pretty good punch. Low level PC's supposedly on their way to catch a ship to Bone Hill stop for rumors in a village bar and end up going into Little Boy Mountain to fight Chagmat. It was a very broad resource but a very handy one at times and I miss it even now.
For 2E this boxed set inspires very mixed emotions among Greyhawk aficionados as it's cool to finally have a boxed set on the center of the campaign world, but Gary Gygax, creator of the whole thing, had nothing to do with this product. Elements of it are there, but it doesn't feel quite like the description in the earlier material or Gary's novels - it feels more like a Forgotten Realms city in some ways. Anyway, it's not the city itself that I am focusing on in this case but the adventure seeds included in the box on cardstock. These are small adventures taking up one 8 1/2 X 11 card front & back. There is usually a small map and some monster stats and the rest is description of what's happening and why. I used some of these cards in every 2E campaign I ran, at different levels, and in different campaigns. There's a low-level one that involves a faerie dragon and some goblins making mischief in a winery, one involving ogres that have been converted to good by a local cleric and what happens when they revert to evil, one about watching a store for a merchant friend while he takes care of some other business, a bathhouse full of frogs, and others that deviate from the typical dungeon full of badguys format. They are nicely different and very easy to drop in wherever needed. A book full of these things would have been an incredible resource but these filled the bill for several years.
For 3E I pulled down OGL stats for the monsters in MM1 and printed them out on index cards and kept them in a box at the table behind my screen. I added more over the years including unique creations with templates and class levels and that's pretty much how I ran my monsters for 3E - I didn't need to stat them out in my notes as I had my cards. I just noted something like 10 Human Bandits Class D, 4 Class C, and 1 Class B and then referred to the cards when it came up. Where this broke down was Dragons. Dragons have a bunch of special abilities including spells that make it nearly impossible to fit onto a card. Even the monster manual entries (my usual fallback for non-carded monsters) don't detail spells as those are chosen individually for each dragon. In 3E they tend to show up more than in earlier editions because of a refocus on classic D&D themes and because there are appropriate versions at all levels They're complex and have huge statblocks too, so a book of prefigured dragons complete with lair maps was a very nice thing to have. It also makes it much easier to include dragons on random encounter tables which I have always liked, especially for cross-country travels. It was a very focused product but a very handy one if Dragons popped up with any frequency in your game.
Yes, I'm looking at games besides D&D too. For d6 Star Wars this book contained Equipment, Vehicles, Ships, Races, Creature stats, Stormtrooper stats, and prominent NPC stats. With the rulebook and this book, you could run a rebellion era game for a very long time. Now a lot of material was added later, but if it was in one of the first 3 movies, this book covered it and that was good enough for most of us at the time. It was a monster manual, arms & equipment guide, rogues gallery, and a race handbook all rolled into one.
For Hero System, mainly Champions, this is a very handy resource. The Bestiary is the best but I'm assuming anyone running a campaign already has that book. This one might be overlooked though as it's nothing but NPC's. Wonderful, pre-statted pre-personality'd NPC's, from cops to crooks to mad scientists to reporters to truckers - all there to be used as contacts, dependent NPC's, hostages, victims, anything, really. They can be dropped in to any modern setting and they will work just fine. They are built at various point levels from incompetent to skilled to low-end heroes to specials like Sparky the Wonder Dog. It's just very handy in actually running a game. Champions is a game where you can make anything but you don't always want to have to do it in the middle of a game and this book helps quite a bit in avoiding that.
For Shadowrun this book contained new contacts, new archetypes, a bunch of maps of things like a subway station and various apartment buildings, and some short adventures that could be dropped in at almost any time. Plus it detailed how credsticks work in the world and how the law codes and equipment worked in the game world too. It was just a big batch of awesome when it came to actually running a game - not advice, not background fluff, but good small scenarios, maps of places shadowrunners were likely to be, and stats for the opposition you might find. I used it well into 3rd edition SR and would probably do it again if I ran a new campaign today.
Finally the Traveller book that saved me the most work, 1001 characters. This book has a bunch of statlines for characters created using the normal chargen system in Traveller Book 1. Seems pretty mundane, right? It's not because it saves actual work and time. Most Traveller fans would probably list 76 Patrons here but I always thought it was overrated as it only gave adventure seeds, not actual adventures. I never had a problem with ideas, but not having maps and stats and details to back them up makes them difficult to use in play on the fly. Supplement 1 however could be used both in advance (Pilot sitting at bar is #45 in S1) and on the fly when one needed stats for an NPC. Yes, I could probably make them up on the fly but I've noticed I tend to repeat myself if that happens too much so I like the unbiased list to pull from rather than handwaving it myself. It's not a flashy or pretty book but it is very handy at the table in play.
Honorable Mentions: For 1E the Rogue's Gallery includes some pregen stats and NPC's. For 3E the DMG includes a lot of NPC sample stats and was used a lot by me in the early days. The 3E supplement "Everyone Else" has stats for NPC types like bartenders and blacksmiths and can be handy at times. The Star Wars Revised Core Rulebook contained stats for a lot of NPC types at various levels from stormtroopers to space pirates to smugglers and was very handy for on-the-fly use. Every Marvel Super Heroes supplement was chock full of heroes and bad guys with full game stats. Every Savage Worlds campaign book comes with full stats for opposition, creatures, and even vehicles - they are very well done. Mutants and Masterminds also has a nice list of opponents from ninjas to cops to robots in a section at the back - all very useful when the night goes off track. Drop in some ninjas and watch the action begin.
As you can see IMO anything that gives me statted up and equipped NPC's, maps, usable detailed adventures, and basically saves me mechanical work during the session rates very highly with me. I like a lot of the theoretical how to be a better DM or monster ecology type books too but in the heat of the game when you're trying to make a world come alive for your players there is no better friend for the DM than prepared material that he doesn't have to prepare.