rsync is an open source utility that provides fast incremental file transfer. (ZPL)
Linux - FreeBSD
Syncthing is a continuous file synchronization program. It synchronizes files between two or more computers in real time, safely protected from prying eyes. Your data is your data alone and you deserve to choose where it is stored, whether it is shared with some third party, and how it's transmitted over the internet. (MPL)
Linux - Windows - MAC OS X - FreeBSD
Unison is a file synchronization program. It is used for synchronizing files between two directories, either on one computer, or between a computer and another storage device. (GPL)
Linux - FreeBSD - Windows - MAC OS X
3 applications