Syncthing is itself free, open-source, peer-to-peer file synchronization — it has no central server at all, exchanging files directly between your own devices over an encrypted connection. It's a fundamentally different architecture from server-based cloud platforms, so the comparison here is really about whether you want decentralized device-to-device sync or a centrally hosted cloud you can also self-host.
If you want shared team access, web-based file browsing, and sharing links rather than just keeping your own devices in sync, Nextcloud is the better fit since it runs as a central server with a full collaboration suite around it.
FAQ
Is there a free, open-source alternative to Syncthing?
Syncthing itself is already free and open source. If you want a centrally hosted, server-based platform instead of Syncthing's peer-to-peer model, Nextcloud is a widely used open-source option.
What's the best open-source alternative to Syncthing?
Nextcloud is the better pick if you need shareable links and a web interface for team access rather than Syncthing's device-to-device-only sync model.
What are the top open-source alternatives to Syncthing?
Browse all open-source alternatives to Syncthing above, or explore the full Collaboration category for more options.