I've noticed a term popping up here and there lately in the OSR world: minigame. No, not those minigames, but smaller, home-brewed rules for covering some aspect of character life that wasn't reflected in the official rulebooks. Telecanter's Port Trading system and Zak's Mass Combat suggestions are two excellent recent examples.
To me, the reason this kind of supplementalism is hitting stride right now stems from a couple of things:
Like I said before, many old-school gamers don't see, beyond a reasonable minimum, the need for niggling consistency, or at least uniformity, in a rules system. The problem with Core Mechanics is that they reduce every possible modelable situation to the same die roll, whether it's a dice pool or a d20. Reducing a search through a port town for goods to trade to one or even a series of d20 rolls loses all of the flavor that searching, stealing, bartering and bickering provides. Transforming all of that flea-bitten nastiness into a sum of dice roll modifiers and a target number is best reserved for people who don't really want to play it out.
(As an aside, this seems to be what Complex Skill Rolls in 3.5 and Skill Challenges in 4e D&D seem to want to be: a way of embracing a different dynamic flow while still reducing everything to a d20 roll. I'm fascinated, but ultimately unconvinced, by this approach.)
But this kind of situation is exactly the challenge-the-player-not-the-character style of play that many in the OSR prefer.
The characteristics I've seen of this type of minigame are:
1. It lends itself to that fabled desideratum, "player agency": as Zak says, the GM can give the PCs "...a choice of choosing the enemy rolled or any tougher one. Choosing the tougher one subtracts from the number of total encounters." In Telecanter's case, it presents a mini-challenge with more information than dungeon exploration typically offers--a role-playing puzzle of sorts.
2. It also lends itself to using/creating random tables, which are both a boon and a curse for the GM. It's a curse if you want to come up with your own tables, but a boon once you have them, because now you can quickly and painlessly generate a batch of content. And there are countless tables available from the web.
3. The tables, however, are optional: one benefit of them is that the players can't game the GM by trying to diving "what Adam would put here". But if they're not available, there's no reason why this style of content generation isn't amenable to good old seat-of-the-pants making it up.
The best point of all of these rules options, however, is that they are short, simple, and above all *optional*. Each one could be hammered out by players and the GM through verbal give-and-take, or
skipped over with a die roll, or otherwise hand-waved away. But the use of these mini-systems, and the selections of what situations to model with them, becomes an element of gamemastering style. And that embrace of individual difference, while still holding to a core set of rules, is why I think there's such a recent explosion of them.