bitpocket is a small but smart script that does 2-way directory synchronization. It uses rsync to do efficient data transfer and tracks local file creation/removal to avoid known rsync problem when doing 2-way syncing with deletion.
csync is a bidirectional file synchronizer for Linux. It allows the user to keep two copies of files and directories in sync. It uses uses widely adopted protocols like SMB or SFTP so that there is no need for a server component of csync. It is a user-level program which means there is no need to be a superuser. With pam_csync, it is possible to create roaming home directories.
Csync2 is a cluster synchronization tool. It can be used to keep files on multiple hosts in a cluster in sync. Csync2 can handle complex setups with much more than just 2 hosts, handle file deletions and can detect conflicts.
Fish is a file syncing application. It can transfer multiple folders between multiple machines, keeping these folders synchronized. The Fish package does not need to be available on each machine in the group, only on the machine initiating the synchronization.
FreeFileSync is a folder comparison and synchronization tool providing highly optimized performance and usability without needless user interface complexity.
freerdist is a version of rdist.
Livesync is an application designed to synchronize data between two systems. If available, it will utilize OS-level filesystem snapshots to ensure data integrity. In addition, it has several built-in functions to allow it to run on all systems in an environment with one master system and one or more slave systems, yet only synchronize data from the master to the slaves, and automatically and reliably determine which system is the master.
pybag is a file and directory synchronization tool that implements a portable bag. You can copy working files and directories into portable devices (as a flash drive or notebook) and work with them.
RDist is an open source program to maintain identical copies of files over multiple hosts. It preserves the owner, group, mode, and mtime of files if possible and can update programs that are executing.
repomirror is a tool to create local mirrors (partial or complete) of Linux distributions and repositories. It currently support Debian derivatives and yum repositories.
Seafile is a file synchronization and collaboration platform. It comes with Dropbox-like file syncing, but is designed to be better suited to teamwork. You can build a file syncing and collaboration service for your team on your servers.
ssync is a contraction of (simple|silly|stupid) filesystem synchronizer.
Super Flexible File Synchronizer
(commercial) Super Flexible File Synchronizer is a tool for keeping your data synchronized in various locations, including backup storage.
SynCE-KDE is a PC / Windows CE connection service application like ActiveSync. It consists of three sub-projects: a full-featured KIOSlave for browsing the PDA (RAPIP), a panel applet for accessing the PDA (RAKI), and a daemon which listens for PDA connections (VDCCM).
syncthing defines a protocol for synchronization of a folder between a number of collaborating devices.
YADSync (yet another directory sync) is a simple tool to easily synchronize a set of directories.