zsync
zsync.moria.org.uk
What Jamie wrote
It has been a long time since I used rsync, but it is a fabulous tool for moving data between machines that you have access to. zsync removes the need for a special server implementation.
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.
If you need to move a reasonably sized asset to a lot of places this is your solution.