Joplin

The sync backend for the Joplin note-taking app, this server gives desktop and mobile clients a shared place to keep notes, notebooks, tags, and attachments, with optional end-to-end encryption for synced content. Clients keep working offline and reconcile with the server when they reconnect, and the same endpoint serves the web client and the mobile apps.

People use it to keep personal or shared notes in sync across devices, and teams use it for meeting notes and shared research vaults.

Hosting the sync server yourself means your notes never pass through a stranger's account, with no per-user subscription.