Figuring out who the cool people are. (Similarly, figuring out what the cool ideas are.) Being able to identify cool people has made my life significantly more awesome than it otherwise would have been, and led me to learn skills like guitar, chess, hiking, rationality subskills and so on that I wouldn't have bothered to or been able to develop on my own. It made high school an amazing experience. It got me laid by hot chicks. It led me to lots of good music. It lets me easily distinguish between meh philosophers/authors/artists and cool philosophers/authors/artists, so I don't waste time. It also alerts me to the existence of communities like LessWrong and the Singularity Institute and to cool intellectual cultures or academic fields like algorithmic information/probability theory (and thus universal AI) and Bayesian computational cognitive science. It lets me know which AI approaches are going nowhere and which might actually be dangerous. It also allows me to feel justified in ignoring people who don't think I'm cool, since I have a good sense for coolness and I think I'm cool, and if others don't have a good sense for coolness then they probably aren't cool and won't become coo...
Memory skills and the ability to do quick arithmetic in your head (the two go hand in hand). I would suggest reading some of Dominic O'Brian's books, and then visit the various mnemotechnic forums. Most of the techniques you will find are geared towards memorizing for competitions, but with slight adjustments they can be used anywhere.
It seems a little silly at first, but it has probably been the biggest return on investment I have ever made. I started practicing these techniques last summer, and when school started I used them (Method of Loci especially - basically you just imagine a spatial location you know well and place images representing the things you want to memorize at unique points in your spatial journey) to memorize as much as I could, using spaced repetition software (mentioned somewhere else on this thread) to lock the most important things in.
Now, instead of writing down copious amounts of notes in class and not understanding a single thing, I just sit, listen, and memorize. I'll also write down broad labels for things I need to make sure I remember (i.e if we are talking about "normal subgroups", I will write down "normal subgroups", but memor...
Programming is great for writing little scripts for yourself, employment, and improving the quality of your thinking.
Self-control is trainable and is applicable to learning and practicing many skills. Small, short and regular training exercises such as writing with your non-dominant hand to write or striving to maintain your posture can be a first step to build it up. (See "Can self-regulatory capacity be increased?" in Heatherton's paper at http://www.dartmouth.edu/~thlab/pubs/11_Heatherton_Wagner_TICS15.pdf.)
How to learn. Using an SRS program has radically changed and accelerated the way that I learn. The SuperMemo website has great instructions on how to formulate knowledge in ways that will best facilitate learning. Using an SRS daily is also a good example of developing a good habit and the influence of this small change can spread into almost every facet of your life.
Politics: sensing motivations, power relationships, coalitions, and knowing how to navigate them safely.
Fermi problems: making a quick order-of-magnitude estimate of a quantity based on rough, but guessable quantities. Not only is this immensely fun, it has many practical applications to quickly rule out unrealistic hypotheses. Solving these in real-life situations (they come up surprisingly often) may also impressive others - your mileage may very, of course.
Dimensional Analysis: either checking the units of a possible answer, or guessing a formula based off the only likely possibility that gives the necessary units. Look at Street Fighting Mathematics sec...
A skill I learned during my brief time as a developmental therapy tech that has transferred well to my current technical support job: the Crisis Cycle from the Mandt system.
Here are the seven phases and responses that therapy techs are trained to take:
How to communicate effectively, in both directions (writing/speaking and reading/listening). It's mostly groups, not individuals that accomplish worthwhile things, and if you can't work well with the right groups (and convince them to work with you), then whatever other skills you have will most likely go to waste.
Learning how to practice. I'm going through some rather rigorous hands-on training where more than half of the students end up failing out. It distresses me to see everyone wasting a full hour running an entire problem when the particular action they actually need help on takes 10 minutes, tops. Practicing efficiently is one of the best things my father taught me. Graduate+ level musicians are bound to have great tips for this. Of course, effective studying is in many ways similar, but it seems more intuitive. That is, no one could possibly be stupid enough to study for a literature exam by sitting down reading the book cover to cover several times.
A bit meta: I realized I had assumed a meaning of "transferable skill" that differed from the one you described in the body of your post (and I had made a comment based on that bad assumption and deleted it). A "transferable skill" would be one that, once you learn, can be easily transferred to others (or so I had assumed).
I think the term you're looking for is "meta-skills" -- skills at improving skills, and what you're asking for here are the most productive/useful/best meta-skills.
Sorry, just a terminology issue, but I think that would be a better search term.
Why are you looking for additional examples? I think it's a good question, but clarifying why you're asking it may lead to better answers. If you're looking to learn transferable skills yourself, then our suggestions can be more targeted. Or if you're looking to make a list of a general audience, then our suggestions can be more general.
To give a very general example: basic mathematics is so transparently essential. Intermediate and higher level math can also be extremely helpful, depending on what problems/activities you're tackling.
As a bit of an offshoot from the people who say math, I'd offer that the transferable skill they're thinking about is how to track the similarities and differences between things. So basically how to make good metaphors.
One idea is to try to become an expert in improving yourself in general. Read up on learning, practicing, getting things done, etc.
-How to use basic hand tools efficiently and correctly (wrenches, soldering iron, etc.)
-Bayesian inference
I'd nominate the standard high school + college math curriculum (algebra and geometry up through the first two semesters of college calculus; multivariable calculus and differential equations are less general). It forms the basis for a lot of science and engineering; one way you know that you really understand something is when you can reduce it to math.
Anything that I've mastered seems to instantly become an easily-transferrable skill.
So, transferable skills: skills that, upon improvement, increase your ability in other areas (and also improve other, higher-level skills).
A basic example would be reading/writing. Knowing how to read and write allows one to access a huge amount of other skills and resources which are otherwise unavailable. A less obvious example would be clear speech (enunciation). Ability to speak clearly improves one's prospects in a lot of different areas (e.g. professional advancement, dating, etc.).
I'm looking for additional examples. Which skills did you find to be most transferable? Did you become proficient in X, and then found this helped you in many other areas of your life? Please share.
(I tried to find whether this was discussed before, and failed; if it was, I would appreciate the link.)