Retriev.er is described as '- GUI for rsync written in Qt' and is an app in the Backup & Sync category. There are five alternatives to Retriev.er for a variety of platforms, including Mac, Windows, Linux, BSD and Cygwin. The best alternative is rsync, which is both free and Open Source. Other great apps like Retriev.er are Truck - Rsync Client (Paid), RsyncOSX (Free, Open Source), cwrsync (Paid) and lsyncd (Free, Open Source).
A graphical user interface for rsync, which looks-and-feels like a standard FTP client, but uses the rsync protocol ‘under the hood’ to boost transfer speeds. In technical terms, it’s an SSH browser with an rsync transfer engine.
cwRsync is a yet another packaging of rsync and Cygwin for Windows. You can use cwRsync for fast remote file backup and synchronization. Rsync uses the rsync algorithm which provides a very fast method for bringing remote files into sync.
Lsyncd watches a local directory trees event monitor interface (inotify or fsevents). It aggregates and combines events for a few seconds and then spawns one (or more) process(es) to synchronize the changes. By default this is rsync.