[corrral]

Apple Notes. Everything else is too high-friction, I always end up back at Notes.app. I always have my phone, that's already installed, and it syncs to every other device I'm likely to want to take or read notes on. I don't do any indexing or cataloging and I don't even bother with categories unless I'm sitting down to write a bunch of notes for a particular project or topic all at once—all I do is make sure the notes are verbose enough and include the right words to let me find what I want later with search.

[EDIT] Uh, so I addressed the how I organize my notes (I basically don't—the value added by organizing them is too low to be worth the effort) but as for the "what": mostly I use notes to capture ideas I don't want to forget. If I'm out and about, or pretty much any time I'm not at my desktop, that's usually some little thought that popped into my head that I'll forget if I don't get it down now (so, that's why I end up back in Notes when I try notebooks or whatever instead—I always have my phone, but sometimes forget a notebook, so I end up with notes both places, at which point it's better to just drop the notebook).

At my workstation, I tend to take notes on things like complex command-line invocations, or else use them as a kind of archive for my paste buffer (for things I may want to copy-paste again in a few days, or months, that may not end up in the shell history where I happen to need them, or weren't anything shell-related to begin with). I will sometimes sketch out e.g. system designs in a notebook, but those get turned into real digital documents before long, if they matter (they don't go in my notes).