Rsync

Rsync is a command-line application used to synchronize directory trees (or single files) across a network. It can be used in a direct, unencrypted fashion ("daemon mode") or it can be used over an SSH transport for added security and flexibility. It can be configured to move all changed files, or it can be configured to only move the delta for the changed file across the network, saving bandwidth at the expense of CPU - very useful for synchronizing across low-bandwidth networks, like the internet.