Treecomp - TreeComp is a utility to keep 2 directory trees and the files within the directories in sync. Download from this site!
arRsync - arRsync is a tool that is designed for macOS that is used for transferring and synchronizing files between a computer and a storage drive.
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.
FileSync - File/folder synchronization utility for keeping files up to date.
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.
CopyQueue - CopyQueue - Copying under control.