Obsidian

Obsidian stores notes as plain Markdown files on your own machine, which means your material stays readable and portable regardless of what happens to the application.

Its organizing idea is bidirectional linking: notes reference each other and the graph of connections becomes the structure, instead of a folder hierarchy decided in advance.

It is free for personal use, extended by a large plugin community, and optional paid sync keeps files consistent across devices without giving up local storage.