Syncthing

Available as a standalone package:

nix-env -iA nixos.syncthing

It can also be enabled as a service. You'll typically want to set the user and the config dir, as with the default values ("syncthing" for the user, "/var/lib/syncthing" for the dir) you won't be able to access the files:

services = {
    syncthing = {
        enable = true;
        user = "myusername";
        dataDir = "/home/myusername/Downloads";
        configDir = "/home/myusername/Downloads/.config/syncthing";
    }
}