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Sounds like Bullet Journal. Will still work in a hundred years. Heck even the offline client could work thousands of years if properly stored^^

Started to use it because of some HN comment suggested it and I am so happy to started this. The only todo system that really stuck with me over time.

That's why I use todo.txt TaskWarrior and org mode are interesting as well. I have to look into differences between then.

I have been using org-mode the last few months and I really like it. It easy to get lost in all the possibilities, but it has extremely cool features, for instance: since org is also a markup language, I can just create my slides with Markdown-like markup and export it to e.g. LaTeX beamer. But since any heading can be made a TODO item, I can just mark slides that are not done as TODO and it will show up on my TODO list.

I've started using Standard Notes, but it kind of feels like it's still beta.

Yup, totally agree. Lesson learned!