Bit Torrent is an interesting new file sharing program. It's been embraced by
3Dgamers.com, and I suspect is gaining in popularity. Essentially, you open a .torrent file and it connects you to everyone out there that is seeding the desired file. When you are done downloading, you leave the window open to let others connect to you. You become a seeder. At some point, you decide that you've helped enough and you shut the window.
But it's a little tricky, because the shared files are specific to various private tracking sites. So when you want to share a file, you have to make a .torrent (a hash file) and indicate which tracking site you want to share through. Sort of. Popular torrent sites are sk8bt and suprnova (<-- no "e").