[URL]http://www.synctus.com/ddar/[/URL]

ddar is a free [B]d[/B]e-[B]d[/B]uplicating [B]ar[/B]chiver for Unix. Save space, bandwidth and time by storing duplicate regions of data only once. Use ddar to:

[LIST] [] [B]Back up local data to a remote server[/B], each time saving space, upload bandwidth and time by only transferring data not already present on the remote server. This is what cloud backup services like Tarsnap allow you to do already. Now you can use your own storage and internal bandwidth to achieve the same thing. [] [B]Back up remote data to a local disk[/B], downloading and storing only changed data each time (the inverse of the above). [] [B]Back up local data to a local external disk[/B], each time saving time and space by only writing data that was not present on the disk already. I have six gzipped tarballs of around 50 GiB each of my laptop stored on my external disk, but all together they are only using 64 GiB of storage. [] [B]Efficiently store any data that has redundancy[/B]. ddar will exploit redundancy across different files stored at any time. [/LIST]

Been messing with this today, pretty damn sweet.