Create Synchronicity is a powerful and lightweight open-source backup and synchronization program; available in many languages.
Syncplicity is a file share and synchronization service that helps businesses share documents or files, manage user permissions and collaborate with external or internal stakeholders across projects.
Rsync is a utility for efficiently transferring and synchronizing files between a computer and a storage drive across networked computers by comparing the modification times and sizes of files.
Unison is a file-synchronization tool that allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other.
GoodSync is a backup and file synchronization software that synchronizes and backs up any files, such as photos, financial documents, and e-mails, between desktops, laptops, servers, and external drives.
FreeFileSync is an open-source folder comparison and synchronization software that creates and manages backup copies of all your important files