Open-source alternatives to Dropbox

2 community-reviewed projects you can use instead of Dropbox.

1. Nextcloud - Nextcloud is a self-hosted file sync, sharing and collaboration platform that lets you store and access files, calendars, contacts and documents on your own server or private cloud. It offers desktop clients for Windows/macOS/Linux, mobile apps, and a large app ecosystem including Talk and online office integrations. (Licence: AGPL-3.0)
2. Syncthing - Syncthing is a free, open-source peer-to-peer file synchronization program that keeps files in sync between two or more devices in real time, without storing data on a central cloud server. It runs on Windows, macOS, Linux, Android, iOS and BSD, with encrypted transfers and a web-based interface. (Licence: MPL-2.0)