Plain text files written using Vim in a Terminal. Then archived by folder to Google Drive or DropBox. I want them to be in a format I, and any machine, can read N years from now. I've been burned before by proprietary solutions ;)

I revisit old files at odd intervals. I have ~5 years worth of notes. What I love most is when I think I have a new idea, or stumbled upon original inspiration, and I find an almost exact sentiment mirrored years ago, albeit using different language. Then I know I've revealed some deep truth that will remain constant for me, and it is only my manner of expressing it that has evolved!