Obsidian - Sharpen your thinking
obsidian.md
Obsidian is the private and flexible note‑taking app that adapts to the way you think.

From ericax’s comment on orange site:

  1. Local-first, Markdown plain text based; 2. Link as first-citizen. 3. As extensible as possible.

Obsidian is a powerful front-end for your knowledge, like an IDE for your notes.