Progress Clocks from Blades in the Dark. They are a weird cross between 4E skill challenges and Apocalypse World countdown clocks, but more flexible than either.
Social Moves from Apocalypse World, maybe with a tad more depth to them. Basically, the DMG system for social checks is a good step in the right direction, but I want something that gives social interactions a little bit more direction.
Intrusions from Numenera. I've never played this game. But I always liked the idea of formalizing the relationship between DM meta-gaming and player meta-gaming in this way. It's similar to Hero Points from Mutants & Masterminds and bennies from Savage Worlds but seems a little more direct.
Descriptors and Trappings from Mutants & Masterminds and Savage Worlds. These are pretty much the same thing. They are a formal system for "DM's Discretion Goes Here." Descriptors are just a wonderful way to encapsulate all the minor, infrequent interactions that aren't important enough to have rules for -- yet, they allow the rules to be phrased in "absolute" terms.
For example, there are a lot of debates about what happens when you cast an electrical spell underwater. There's no rule that says it might work any differently underwater than on land, so a strict RAW purist would not modify the spell's effects. Many other players would say: no, the DM can always modify how things work in special circumstances. But is this circumstance special? It may come as a surprise to the strict RAW purist!
With Descriptors, the DM can say, "ah, that spell has the lightning descriptor, so it's going to interact with this environment's water descriptor." This is a constant reminder that the DM's discretion might come into play, while also providing guidance to that discretion. The benefit is purely psychological but I've found it very helpful in running games smoothly.
