Jigsaw Download, or short jigdo, is a tool designed to ease the distribution of very large files over the internet, for example CD or DVD images. Its aim is to make downloading the images as easy for users as a click on a direct download link in a browser, while avoiding all the problems that server administrators have with hosting such large files.
Article about jigdo on heise.de (c't-Magazin; German)
This software is now in "maintenance mode", development has stopped. Unfortunately, this means that various parts (in particular the GUI download tool) will remain unfinished.