FreeFileSync

DirSync Pro is a cross-platform, open-source file synchronization and backup utility. DirSync Pro can be used to synchronize the content of one or many folders recursively.

Seafile is an open-source, self-hosted file sync and share solution with high performance and reliability. Sync, access, and collaborate on files on your own server or private cloud.

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.

Create Synchronicity is a powerful and lightweight open-source backup and synchronization program; available in many languages.

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.