Obsidian is a free note-taking app that stores Markdown notes privately on the device and links them into a personal knowledge base.

Linked notes - Connects notes with internal links backed by plain Markdown files that work offline.
Graph view - Visualizes relationships between notes to surface patterns.
Canvas - Arranges notes, media, and diagrams on an infinite board for research and brainstorming.
Plugins and themes - Extends the app through thousands of community plugins and themes plus an open API.
Sync - Keeps notes across devices with end-to-end encryption, version history, and shared vaults.
Publish - Turns notes into a fast online wiki or knowledge base with custom domains.

It is used by writers, researchers, students, and teams keeping personal notes, journals, documentation, and project knowledge.

The app is free without limits. Sync and Publish are paid add-ons.