There is apparently also a "Donation Edition" -

    Everyone who donates is eligible to download a special 
    version of FreeFileSync without any advertisements,
    including a few of bonus features.

It is built from a different source, so it's neither free or open source. Not that anything wrong with it, but it should probably not be distributed under the same name - https://freefilesync.org/faq.php#donation-edition

chaosite

This project is kinda wonky from a free/open-source perspective. It's ostensibly licensed under GPL-3 (i.e., there's a source zip which has a License.txt that's GPL-3), but it otherwise acts like freeware with a single developer.

It has a history of bundling file-droppers/malware; there's a donation edition with a different feature set, with some extra features (including removal of arbitrary limitations on the regular versions); the installers are binary blobs, and there's no attempt (and passive hostility) towards integration with distros and package managers; source control isn't provided, and there's basically no attempt to create a dev community.

The binary installers may or may not include things that are not in the provided source code, like installer and ad systems. That's not really cool.

justinclift

It seems like it serves the same kind of purpose as SyncThing (https://github.com/syncthing/syncthing).

SyncThing seems like a safer bet (better community, proper OSS, etc). :)