Soulseek is a peer to peer client, and it rocks. You can find a lot on SLSK. I usually go to slsk to find my music because, like smith said, it's hard to find more obscure music torrents. And sometimes I just want a sample or song, not the whole album and every album they ever made or were ever featured on.
I use torrents to download big files. The client I use is BitComet, which I think is pretty sweet, but I haven't used the others.