Obsidian

Obsidian is a note-taking app that stores everything as plain text Markdown files right on your device. It is built by a small independent team and is especially popular with people who care about privacy and want full control over their data. It suits students, researchers, writers, and founders building their own knowledge base.

You write notes just like in a text editor, except you can link them together to form your own network of ideas. A visual graph shows how individual notes relate to each other and helps you discover connections you would otherwise miss. Everything runs locally, so you need neither an internet connection nor an account to work.