Treecomp - TreeComp is a utility to keep 2 directory trees and the files within the directories in sync. Download from this site!
bup - Very efficient backup system based on the git packfile format, providing fast incremental saves and...
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.
Duplicati - Free backup software to store backups online with strong encryption. Works with FTP, SSH, WebDAV, OneDrive, Amazon S3, Google Drive and many others.
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.
Duplicity - Duplicity backs directories by producing encrypted tar-format volumes and uploading them to a remote or local file server.