Notesnook
notesnook.com
A fully end-to-end encrypted, open-source note-taking app with rich text editing, offline support, and cross-platform sync.
Why it is useful
Combines the editing experience of a modern note app with zero-knowledge encryption — the server literally cannot decrypt your notes. Unlike most encrypted note apps, it doesn't sacrifice formatting or usability for security. The open-source codebase allows independent security audits, which is rare in this category.