Unison is a file-synchronization tool for Unix and Windows. It allows two copies 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. Unison can deal with updates to both replicas of a distributed directory structure. Updates that do not conflict are propagated automatically. Conflicting updates are detected and displayed. Unison can communicate through a direct socket link or through an rsh/ssh tunnel. It uses network bandwidth efficiently.

http://www.cis.upenn.edu/~bcpierce/unison/

This can be used to to synchronize files within multiple web servers in a load balanced server setup.