Some perhaps-too-obvious ones: comparative advantage, arbitrage, Schelling point, plurality, transfer (in education), deliberate practice, tacit knowledge.

So many! Just a few that come to mind: compound interest, observation selection effect, bias, differential technological development, function, present value, correlation, subjective experience, trivial inconvenience, self-amplifying capacity, expectation, etc.

Also, many conceptual distinctions in philosophy are useful: analytic-synthetic, a priori-a posteriori, de re-de dicto, phenomenal-intentional, proximate-ultimate, intrinsic-instrumental, factual-normative, contingent-necessary, etc. Taxonomies of views in a given field, too, can be quite helpful in conceptualizing the relevant boundaries (e.g. Chalmers on views about consciousness or Miller on views in metaethics).

I guess one could say the concept of a useful concept is itself quite useful.

Most functions are not linear. This may seem too obvious to be worth mentioning, but it's very easy to assume that various functions that appear in real life are linear, e.g. to assume that if a little of something is good, then more of it is better, or if a little of something is bad, then more of it is even worse (apparently some people use the term "linear fallacy" for something like this assumption), or conversely in either case.

Nonlinearity is responsible for local optima that aren't global optima, which makes optimization a difficult task in general: it's not enough just to look at the direction in which you can improve the most by changing things a little (gradient ascent), but sometimes you might need to traverse an uncanny valley and change things a lot to get to a better local optimum, e.g. if you're at a point in your life where you've made all of the small improvements you can, you may need to do something drastic like quit your job and find a better one, which will temporarily make your life worse, in order to eventually make your life even better.

The reason variance in financial investments matters, even if you only care about expected utility, is that utility isn't a linear function of money. Your improvement in the ability to do something is usually not linear in the amount of time you put into practicing it (at some point you'll hit diminishing marginal returns). And so forth.

I never really consciously thought about the idea of status until I came across it in this forum. It helped me identify and understand some behaviors in myself and others which puzzled me before.

Regression to the Mean

In usability, the concept of Affordance is pretty useful.

From off site:

Energy and Focus is more scarce than Time (at least for me), Be Specific (somewhat on site, but whatever),

From on the site:

Mind Projection Fallacy, Illusion of Transparency, Trivial Inconveniences, Goals vs. Roles, Goals vs. Urges

"Shibboleths," said Will Newsome, somewhat passive-aggressively.

Extracting examples from some of my past comments: proving too much, selection bias, Nash equilibrium, denotation & connotation, insight & intuition as recognition.

Others from game theory & economics: free riders & hold outs, the tragedies of the commons & the anticommons, precommitment, coordination games, average-marginal confusion, thinking at the margin.

Two more that're a bit Sequences-esque but which I like so much and use so often I'll highlight them anyway:

• Reference class forecasting. It doesn't just help one beat the planning fallacy by predicting durations; it can predict probabilities too, and one can apply it to other people as well. Will a flaky friend show up to a meal? Run a reference class forecast. Am I likely to get a "yes" if I ask someone for such & such a favour? Run a reference class forecast. How likely is it that a claim an acquaintance has just made is true? You get the idea. (Guess I should throw in reference class tennis as well. Fortunately, just as with actual tennis, it's hard to play reference class tennis on my own, so reference class tennis isn't too big a risk when I do solo reference class forecasts.)

• The typical mind fallacy, which I deliberately use as a heuristic. If I'm trying to explain someone else's behaviour (or figure out an aspect of someone else's psychology more generally), I use myself as a model and ask myself why I might do the same thing. This may give an incorrect answer, but it gets me an answer faster than trying to derive someone else's behaviour from first principles, and I can always introspect further to guess how well my self-derived answer might generalize to others.

Slightly less well-known cognitive biases: just world fallacy, mean world syndrome, and that one bias that really needs a name where people underestimate mundane risks and overestimate dramatic risks.

Statistics: skewness (and medians as outlier-&-skew-resistant averages), standard error (and error bars more generally), systematic error vs. random error, effect size, power, meta-analysis, bootstrapping & Monte Carlo methods, sampling methods.

Credibility also can be useful. Most importantly: Are the threats, precommitments and offers you make credible? Could and would you actually go through with them if you found yourself in a situation where the conditions you stated are fulfilled? If you arrange the exchange in such a way that acting on your words imposes a low cost (better yet: no cost) on yourself, you'll gain lots of bargaining power.

A quick example: When educating children, misbehaviour has to have consequences. Now, you have to choose these punishments in such a way that they impose little organizational and emotional cost on yourself while being serious enough that kids want to avoid them (but, of course, also not too serious ;)). If done correctly, you'll have to punish a few children a few times, but then they will have learned. If done incorrectly, you continuously threaten with punishment, but there is no clear line where you have to act, and put in such a situation you don't even want to punish them, so the children will continue to misbehave.

Fail-fast.

Robin Hanson's marginal charity has been quite useful for me.

Especially, when I use it for doing marginal charity towards myself. Example: I'm eating a slice of cake. The difference in utility for me between eating 3/4ths of it and all of it is small. So if I leave a 1/4th of it for future-me, the difference in utility for future-me is much larger since it is the difference between having no cake and having some cake.

The concept of privilege of the "check your..." variety. It's not without it's problems as a tool - it can too easily be used as a Fully General Counterargument - but it's an important thing to be aware of and probably the single concept I've learned in the last two years that has most changed my outlook on the world.

The tradeoff between efficiency and accuracy. It's essential for computational modeling, but it also comes up constantly in my daily life. It keeps me from being so much of a perfectionist that I never finish anything, for instance.

Various bits from Dennett: cranes versus skyhooks, the Cartesian Theatre, 'figment', the intentional stance, the notion of an intuition pump.

Braitenberg's Vehicles does something indescribably non-verbal. It's a tiny book that passes through your mind silently, like a ninja, and then any lingering shreds of vitalism you have just sort of explode and blow away on the wind. Every child should have a copy (and robot parts to use it with).

Cost of delay. If you have two projects which when finished will generate (say) the same amount of value per time and it takes 2 weeks to do the first and 1 week to do the second, by sequencing them in the other order you gain 1 weeks' worth of value. If instead you interleave them, working "in parallel", with no real gains in total time, you lose 1 weeks' worth of value. Also and more obviously, if one project generates way more value per time, finish it first. If a project loses value while not finished, consider finishing it first. (for example taking an idea to market - your market research undergoes depreciation)

[META]

I suggest moving “repository” threads to Main, because they are threads where it would still make sense to add new entries arbitrarily long after they are originally posted, and ISTM that new comments in old threads are more likely to be noticed in Main than in Discussion.

Both positive and negative black swans . Aditionally: randomness and regression to the mean.

Moral Foundations theory (all moral rules in all human cultures appeal to the six moral foundations: care/harm, fairness/cheating, liberty/oppression,loyalty/betrayal, authority/subversion, sanctity/degradation). This makes other people's moralities easier to understand, and is an interesting lens through which to examine your own.

The Big Five Personality Traits - though I've heard these don't seem to fit non-Westerners very well. Probably still useful when thinking about Westerners. (For example, when evaluating someone as a romantic partner or a business partner in some risky venture, I find it useful to deliberately consider their neuroticism. Or when considering suggesting someone try traveling or anything adventurous, their Openness To Experience is probably relevant.)

A teleological, non-reductionist worldview, supposedly traceable through Plato, Aristotle, Augustine, and Aquinas is wrong, but is a useful concept to be aware of because some people think it's correct. It's related to why some people, particularly some religious people, oppose homosexuality. Edit: I should add that I'm not recommending an in-depth study of this concept, just reading a few blog posts on it, and then more if it's interesting to you or if you really need to engage with believers for some reason.

Supervenience. For example, "the mind supervenes on the brain." The idea here is that a mind cannot be different without the corresponding brain being different as well.

Here's a bunch of useful business concepts, as put together by Investling.

I have a list of ~500 concepts / "patterns in reality" that I think it would be useful to have "handles" to, by having words/phrases for them or just having them as mentally-cached concepts. (Once I started actively looking for such concepts I found quite a lot of them.) I'm planning to someday-soon sit down and actually make up words/phrases for each of these; I've made up words/phrases for 37 of them, and my notes-to-self are already peppered with them. Unfortunately I don't have the time right now to put them all here, but I probably will when my notes are more organized (like, when I've actually made up the words).

One example of such a concept: "imposing conditions that would have been evidence about optimal behavior in the EEA, with the intent of making oneself more prone to that behavior" - I call this "savannah-signalling" and it comes up in a lot of places.