If you're looking for music,
Soulseek is what you want. There's more than music on there, but it really is the meat of what is shared. The thing that makes this program better than any other music sharing program (or just P2P software in general) is that you can search through users' shared folders. An invaluable tool in finding obscure and new music.
Warez and such, yeah, Bitorrent. User interaction with it blows though from my experience. It does work well, I just don't like
how it works.
Another alternative which probably isn't too widely known is
Hotline. It uses trackers like Bitorrent, but it tracks servers and the files on them, not users. So it's basically like a searchable network of FTPs with more features. Only problem is, you have to have an
in, so to say. Public servers usually require uploading while downloading and/or are just plain slow, but private servers expect good uploads or money for an account. Barring that aspect of it, it's an excellent program for getting anything you want.