The most popular open-source software that I maintain at the moment is the rsync file transfer program. This program uses a very clever checksum algorithm that allows it to update a changed file on a different system by just sending the differences between the files. The result of this is that you can send data more quickly over a slow connection (at the expense of using more disk I/O and CPU).
This program comes installed on most modern Linux/BSD/MacOS/etc. systems, so chances are you may already have it. If not, the link above will direct you to where you can download it.