Conduit - A GNOME application to synchronize files, photos, emails, contacts, etc. between computers or devices.
DirectorySynchronize (DirSync) - A lightweight, customizable application to sync files between many different types of devices.
FullSync - Open Source File Syncing Software - A customizable data backup and synchronization package.
iFolder - A cross-platform file and folder synchronization application developed by Novell that allows for data sync between computers with different operating systems.
JFileSync - Application that syncs folders on the same computer, different computers, and different devices.
OpenSync - Syncs personal information management (PIM) data (contacts, calendar, tasks, notes) between PCs and mobile devices.
PowerFolder - Secure synchronization, backup and sharing of files for non-tech savvy users.
rsync - Command line file and folder syncing, requires initial configuration. For tech-savvy users who like the command prompt.
Unison - Developed by the University of Pennsylvania, it works on Windows, Mac, Linux, and Solaris.
WinSCP - Handles semi and fully automatic directory syncing. Also an FTP program.