Jigdo

Jigdo is a neat little download program that pulls pieces of a larger file off the web from HTTP or FTP sources and assembles them into a complete image on your hard disk. It was originally developed to assist in downloading Debian CD-ROM iso images, but to reduce the load on the major Debian servers by distributing that load to other servers.

Special features
What makes Jigdo unique in comparison to other download progams is that the smaller pieces it downloads are actual files that are in the content of the larger file (it supports multiple "container" formats, including ISO 9660, UDF (CD-ROM/DVD), and tar archives). That means that the files can be normally put on remote sites, but the whole image doesn't have to exist on the remote site. This also allows the separate files to be downloaded individually without jigdo. But jigdo does produce a full exact image by adding the other data of the image (header, content list, boot block, etc.) to be included in a separate jigdo file that will be downloaded.

Jigdo also allows "upgrading" an image -- if a modified image is released, only the new data will be downloaded.

If a download is interrupted, jigdo can pick up right from where it left off.