DeltaCopy is an open source, fast incremental backup program. Let's say you have to backup one file that is 500 MB every night. A normal file copy would copy the entire file even if a few bytes have changed. DeltaCopy, on the other hand, would only copy the part of file that has actually been modified. This reduces the data transfer to just a small fraction of 500 MB saving time and network bandwidth.
DeltaCopy is a "Windows Friendly" wrapper around the Rsync program, currently maintained by Wayne Davison. "rsync" is primarily designed for Unix/Linux/BSD systems. Although ports are available for Windows, they typically require downloading Cygwin libraries and manual configuration. http://www.aboutmyip.com/AboutMyXApp/DeltaCopy.jsp