Obsidian

Obsidian is a powerful, highly customizable note-taking and knowledge-management application designed to help users build a "second brain." Unlike many other note-taking apps that store data in proprietary formats on the cloud, Obsidian operates on top of a local folder of plain-text Markdown files. Your thoughts are yours. Obsidian stores notes privately on your device, so you can access them quickly, even offline. No one else can read them, not even us. Your mind is unique. With thousands of plugins and themes, you can shape Obsidian to fit your way of thinking. Your knowledge should last. Obsidian uses open file formats, so you're never locked in. You own your data for the long term.