Ellié Computing Merge synchronizes directories thanks to a system of simple rules. You can choose to realize a mirror toward left side from the right side for example, or you can make an update taking only more recent files.
The synchronization works between any two kind of connection. You can synchronize two FTP sites together, two folders in local network, a local folder with a FTP/S site and even Subversion repository toward a FTP site.
See also: ECMerge documentation: Folder Comparison and Merging