If you think BitTorrent is about pirating music, you're missing out on a really cool emerging distribution system1. Carl Franklin uses BitTorrent to distribute DotNetRocks. We've used it on the Monoppix project2 to distribute over 1.7TB in 12 days on a $5/month hosting package (there are 620 downloads of a 445MB file in progress right now). It's an economical and efficient way to distribute large files.
Peter Provost points to a java applet BitTorrent simulator which shows why this is so cool. Fire it up, hit the "+" key 10 times (ten people want the file), then hit the "s" key twice (two seeds to supply the file). Watch the fun - at first, the seeds send bits of the file out to some of the downloaders, but then the downloaders start distributing the bits they've got with eachother, and the bits start flyin:
1In fact, trading
copywritten music or video over BitTorrent is a pretty dumb idea, since it's not
at all anonymous. Users are visible and trackable.
2We're using BlogTorrent for Monoppix. It doesn't have all the bells and whistles of some of the other trackers out there, but it's a simple PHP/MySql system that installed in 2 minutes and works well.