zsync is a file transfer program. It allows you to download a file from a
remote server, where you have a copy of an older version of the file on your
computer already. zsync downloads only the new parts of the file. It uses the
same algorithm as rsync. However, where rsync is designed for synchronising
data from one computer to another within an organisation, zsync is designed for
file distribution, with one file on a server to be distributed to thousands of
downloaders. zsync requires no special server software — just a web server to
host the files — and imposes no extra load on the server, making it ideal for
large scale file distribution.

zsync is open source,  distributed under version 2 of the Artistic License.
Feedback,  bugs reports and patches are welcome