I make a folder called "notes" on any given machine and make a new text document inside of it when I have a note I want to make. I know this isn't clever and "it doesn't scale!!!" but I have multiple computers and it's easy to merge the notes into a single backup, and I always know where to go for the notes I take.

If I need to share a note, I use a github gist but it's also just a linear text storage so I'll prolly replace it with something else at some point...