I was contemplating writing an entry about the one-day interviewing class/workshop that I took at SGI in the mid-'90s, but it turns out I've previously written up a fair bit of that, in a comment here a couple years ago about conversational techniques I learned from it. I may promote that to a full entry at some point, and I may also write up some of the how-to-conduct-an-interview parts of it, but thinking about that sparked another thought.
Sumana recently made an offhand comment to the effect (iIrc) that that interviewing class had a big effect on my life. And I don't think I usually think of it that way, but she was right. It made me a much much better interviewer at work (I'm still far from perfect, but I've gotten praise for my interview writeups), but more importantly, it gave me tools with which to build a useful framework for being able to interact successfully with people I don't know well. (The short version of that framework: ask a lot of questions, and be genuinely interested in the answers. I already had the be-interested part; I just don't think I had given any significant thought to the questions part before that class.) (Disclaimer: This paragraph is not meant to imply that anyone else ought to interact the way I do. Just describing what works for me.)
And just now, remembering Sumana's comment, it occurred to me that there was another class that had a disproportionate effect on my life: the six-week summer-school typing class that I took sometime in high school. I was going to say that may've been the single most useful practical skill I learned during high school, but I suppose driving is way up there. (Though I probably learned more about that from my father than from driving class.)
I don't remember my frame of mind going into either the interview class or the typing class, nor do I remember what I hoped to get from either of them. But I certainly didn't expect them to have such far-reaching effects.
The high school stage tech class that I mentioned here a couple weeks ago was also life-changing, but that was because of who I met in it, not because of what the class taught.
I hasten to add that I've had some really wonderful academic classes, including some that have given me new paradigms and new tools for thinking about the world, taught by some awesome teachers. But in terms of effects on my life, I think the interview class and the typing class are the big two.
So now I'm curious: what classes have you taken, if any, that really changed your life? You can define life-changing any way you like; for example, my focus in this entry wasn't on intellectual or emotional (added later: or physical) change, but your answers can certainly involve such changes.