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.
Skill 'transference' is in fact the generally accepted term for kalla's topic. http://en.wikipedia.org/wiki/Transfer_of_learning
edit: Holy Kerning B-Man!
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.)