Syncthing

Syncthing takes a fundamentally different approach from every other tool in this category: there is no central server at all. Files sync directly, device-to-device, over an encrypted connection — your laptop talks straight to your home server or phone, discovering peers via a public relay/discovery service but never routing your actual file contents through it. For anyone whose real objection to Dropbox is "why does a third party need a copy of my files at all," Syncthing is the most literal answer: nobody but your own devices ever holds the data.

Since its first release in 2013, it has grown into one of the most-starred open-source projects in this space (86k+ GitHub stars), with continuous, active development. Folder sync is selective per device, changed and deleted files get proper versioning (staggered, simple, or trash-can strategies), and everything is configurable through both a clean web GUI and a full REST API for scripting.

The honest limitation is scope: Syncthing is not a Dropbox replacement for sharing files with people outside your own device set. There's no web-based file browser, no public share links, no password-protected file requests, and no concept of inviting an external collaborator into a folder — every device in a sync group has to be one you personally control and configure. Devices also need to be online (directly or through a relay) at the same time for a change to propagate, unlike a server-based tool where you can push a file any time and the recipient pulls it later. For personal multi-device sync with zero server trust and zero cost, nothing else on this list matches it; for team file sharing with outside collaborators, look at Nextcloud, Seafile, or Pydio Cells instead.