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 section "Dimensions" for some great applications.
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.)