zsync

client-side implementation of the rsync algorithm
zsync is a file transfer program to download files from
remote web servers. If a previous version of a file is available
locally, zsync will only download changed parts and hereby
minimise the download volume. The algorithm is the same as used
by rsync(1), but zsync does not require any server software
(apart from a web server), nor does it need shell access.
Instead, it uses a control file (.zsync file) that describes the
file to be downloaded, which it uses to determine the blocks to
fetch. This file is created once on the server (and not for each
request) and sits next to actual file to download