Jdownloader is a popular, open source, multi platform package for downloading media from sites in bulk. It's noteably the only way to download youtube videos that doesn't suck.

It's an incredibly large project (it has it's own wikipedia page and is downloaded almost as much as VLC in certain countries).

Source:

http://beta.jdownloader.org/developmentquicktutorial

(it uses some sot of quirky SVN thing so they have a tutorial on how to use it there)