When I think of "settings" for D&D-type games, I think of two different sorts of products.
One is the sort in which an entire campaign may take place, detailing in broad strokes a fairly large geographic area. Many settings of this type have been published, and you probably know at least a few of them by name if not more intimately: Forgotten Realms, Greyhawk, Dragonlance, and the Known World (a.k.a. Mystara,) among many, many others. With this type of setting, you generally get a map, a description of the various regions, the nations and cultures, major NPCs and factions, history and legends, and probably some adventure hooks and a few new character classes or sub-classes.
It's the second kind that I want to talk about today. Exemplifying this type of setting is B2: The Keep on the Borderlands. While it (and most other examples of the type) was published as an adventure module, it's really a mini-setting, without an explicit plot or any of the other common trappings of most adventure modules. Instead, we get a very detailed base of operations in the titular Keep, a small-scale wilderness map and a few outdoor encounter areas, and a sprawling map of the Caves of Chaos. All areas of Keep and Caves are fully stocked, with stats for all inhabitants. Each area is described in detail, with furnishings and treasure. Relationships between the factions of monsters are described, with hints on how they might interact and how the players can manipulate them to their benefit. A table of rumors provides adventure hooks, which the players may pursue or not. There is no predetermined goal, no "win" or "lose" conditions, nothing expected of the player characters but to go forth and explore something.
In a way, a micro-setting is like the fabled megadungeon in that it's meant to be visited again and again, changing with time and the characters' actions instead of appearing once and never seen again. Unlike the megadungeon, the micro-setting isn't meant to sustain an entire campaign on its own, though a phase of a campaign may center around it.
There's a lot to love about these little settings.
Having pre-mapped and stocked micro-settings without plots means I can run whatever I like, or whatever the players want to pursue. A good mini-setting has the potential for a variety of different kinds of adventures. The Keep, for instance, has opportunities for scouting, rescue, search-and-destroy, and exploratory missions, service to good and noble causes, and treachery and betrayal. It's all there, and my group can pursue whatever they like. I won't have to scrap some elaborate adventure I've written up for the night's session nor improvise something more to their liking completely off the top of my head. If the players decide they'd rather investigate where those skeletons and zombies are coming from instead of searching for the merchant's wife, I'm prepared and ready to roll with it.
Re-using familiar locations gives a campaign continuity, which makes it more believable and gets players more invested in it, in a way that making new locations from scratch for every adventure doesn't. Having recurring locations, characters, and factions figure into each new adventure makes the world feel real and organic, not just a series of vignettes unconnected by any common thread but the same protaganists. Instead of inventing a new mercenary outfit to support the next villain in his plot to overthrow the castle, you can use the goblin tribe the players ran out of the cave complex three sessions ago. The goblins have a reason to hate the PCs, and the players have an emotional stake in proving that their first victory was no fluke. Plus, they get to use whatever they've learned about these goblins' strengths and weaknesses, which makes their past encounters meaningful to current events, instead of just war stories to tell around the tavern.
You can build a campaign world from the bottom up by stringing together several micro-settings. It's a process of discovery from both sides of the screen. Players discover the world by playing in it, and you discover how each micro-setting relates to others to form the greater world. All the bits in between micro-settings can be fleshed out as-needed rather than set in stone in advance. Without the overarching plan that a detailed macro-setting imposes on the campaign, you're free to build your world on the fly for the greatest enjoyment of your group. What's beyond the dark forest that they players have declared their intention to cross? Not some generic village that you put on a map because it looked like there should be something there, but another micro-setting bubbling with adventure potential!
You can use a mix of your own micro-settings and published ones. I don't know about anyone else, but it can be creatively liberating for me to drop a micro-setting ready-made by someone else into my game and figure out how to put it to use. Because it's sprung from a different mind than my own, it forces me out of unconscious patterns, but because it's a micro-setting full of possibilities and not a story with a predetermined plot, my imagination is set free to fly on new courses rather than simply put in a different straitjacket.
Using other people's micro-settings in your campaign also reduces prep work for you while still allowing maximum detail. If the players decide to explore that ruined keep looming on the hill, you've got it all mapped and stocked, and if not, you haven't spent a ton of time writing it up for nothing. With a mixture of your own creations and published works, you can have a ton of very detailed areas with a fraction of the effort it would take to do it all yourself.
Unfortunately, the supply of micro-settings available seems to be a lot less than that of macro-settings and plotted adventures. If you know of any other good micro-settings, feel free to post titles and links in the comments.
Next up, I talk about the ingredients that make a good micro-setting.