SyncToy!

FreeFileSync is a free Open Source software to synchronize files and synchronize folders available for Windows, Linux and Mac OS X. It is designed to be easy to use while having nice visual feedback along the way.

GoodSync is the high-end professional easy and reliable file backup and file synchronization software.

SyncBack is a backup and file synchronization software that allows backing up and synchronizing files to the same or different drives, to different media (CD, DVD, Flash, zip), or to a remote server.There are three versions of SyncBack available: SyncBack Freeware, SyncBackSE (shareware) or SyncBackPro (shareware).

Simple fast backup for Windows. At a glance - small and very lean, very fast delta copier, excellent user interface, device tracking, shadow copying, move/rename detection, run-as-a-service mode, robust support for multi-million item backups with terabytes of data, intelligent and discrete update mechanism. Ideal for backups going to external USB drives and NAS devices.

Grsync is a rsync gui, a graphical user interface for the rsync file synchronization and backup tool. It works on linux, solaris, windows, mac os x and maemo. It uses the gtk libraries and can work under the gnome desktop.

Pulse, officially-sanctioned fork of Syncthing , is a free, secure, and distributed file synchronisation engine.Synchronise your files without also sharing them with a stranger in the cloud.

Create Synchronicity is a nice, lightweight backup program with a very user-friendly - yet powerful - interface, that synchronizes files and folders extremely quickly (it can even recognize paths such as "MyUSB"\MyFolder, and locate the USB drive called "MyUSB").Its open source, fully portable, and multilingual ; and it can schedule backups and has many advanced options like DST correction and regexp settings.

Allway Sync is a file and folder synchronization software for Windows.

DirSync Pro (Directory Synchronize Pro) is a small utility for file and folder synchronization without bidirectional file-deletion. DirSync Pro can be used to synchronize the content of one or many folders recursively. In difference to Allway Sync, SyncBack or Synkron in the bidirectional synchronizing mode no files can be deleted!Using DirSync Pro you can make incremental backups. Only new/modified/larger files would be copied.Use DirSync Pro to easily synchronize files from your desktop PC to your USB-stick.DirSync Pro is programmed completely in platform independent Java. »

A graphical interface for the Rsync engine that will synchronise files between two computers either locally or over the internet. The beauty of Rsync is that it only transfers the exact part of a file that has changed (differential synchronisation), saving bandwidth and time.

SyncFolders helps you to synchronize the contents of two folders, including any subfolders. The folders may be on different disk drives, on the network, or storage devices attached to the USB port, including external hard disk drives, flash drives, and any other type of USB storage device. The program allows you to easily copy new and updated files and folders in either or both directions. SyncFolders is a very useful tool for backup and synchronization purposes.