It doesn't have to be from a completed source. You can download bits of files from different users until you have the whole thing. And all the while, others can get whatever you have of it. It stores cumulatively in a temporary folder, then just transfers to another one when it's completed. And then people can keep getting whatever they need from you as long as it's on your computer, of course.
I know it's not new, but there are a lot of things available through eMule that can't be found with bittorrent. That's the main reason why I'm mentioning it. Plus, its popularity seems to be at an all-time high now, meaning tons of people using it all the time.