Dropbox is published by Dropbox. This page compares only projects with an explicit relationship to it; shared categories alone never create an alternative claim.
Recommended order uses OpenAlternative organic alternatives v4 (version 4). Relationship confidence, category fit, logarithmic popularity, as-of maintenance and freshness, bounded anomaly quarantine, and known-data penalties. Payment is fixed at zero.
A hard constraint removes a Project only when its stored fact is known and conflicts. Unknown facts remain visible, are called out below, and never become a negative claim.
Continuous file sync between your own devices only
Syncthing synchronises folders directly between devices with no server in the middle. There is no account, no storage quota, and no third party holding the data, because the data never leaves the set of machines you nominate.
Self-hosted photo and video backup for your own server
Immich replaces a cloud photo service with software that runs on hardware you own. Mobile apps for iOS and Android back up new photos and videos in the background, so the library on your server stays current without you thinking about it.
File sync, sharing, and a suite of apps on your own server
Nextcloud started as file sync and grew into a full collaboration suite: documents, calendar, contacts, chat, and an application ecosystem on top of the same account system.
Groups appear only when at least two credible projects qualify.
Syncthing, Immich, Nextcloud