rsync - rsync is a file transfer program for Unix systems. rsync uses the "rsync algorithm" which provides a very fast method for bringing remote files into sync.
FreeFileSync - FreeFileSync is a free open source data backup software that helps you synchronize files and folders on Windows, Linux and macOS.
GoodSync - GoodSync provides highly reliable file backup and synchronization for both individuals and businesses.
SyncBack - Backup software for the PC: powerful, easy to use solution for backing up and synchronising files on a local/network drive, the cloud, FTP server and more.
DirSync Pro - DirSync Pro: Powerfull synchronization and backup utility. It's portable, easy to use, open source, 100% free, 100% spyware-free, no limitations
FileMenu Tools - FileMenu Tools lets you customize the context (right-click) menu of Windows Explorer.