Duplicati - Free backup software to store backups online with strong encryption. Works with FTP, SSH, WebDAV, OneDrive, Amazon S3, Google Drive and many others.
Treecomp - TreeComp is a utility to keep 2 directory trees and the files within the directories in sync. Download from this site!
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.
GoodSync - GoodSync provides highly reliable file backup and synchronization for both individuals and businesses.
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.
Syncthing - Syncthing replaces proprietary sync and cloud services with something open, trustworthy and...