Rsync
rsync is a program that syncronizes files between two sources. The two sources may be local files on a single computer or one source may be on a remote computer. When operating locally rsync communicates via filesystem calls. When operating between a local machine and a remote host, rsync communicates either via its own unencrypted protocol on port 873 or via an rsh/ssh tunnel.
The original implementation of rsync is a unix command line program that does not work on windows without cygwin.
Alternative and derivative projects
xdelta
A binary diff program based on the rsync algorithm.
DeltaCopy
http://www.aboutmyip.com/AboutMyXApp/DeltaCopy.jsp
A reimplementation of rsync in windows.
RsyncCE
http://www.rainer-keuchel.de/wince/rsync.html
PocketPC port. I will probably write more about this soon.
Unison
http://www.cis.upenn.edu/~bcpierce/unison/
A cross-platform, bi-direction replication tool. Features GUI or non-GUI operation.
Robocopy
Windows file/folder replication... now standard with Vista.
http://en.wikipedia.org/wiki/Robocopy