Kazza and limewire are a couple I use. Kazza is the armpit of the P2P programs. It is a full of worms and virus's that basicly kill your computer. So unless you like to reformat every month I would recomend to stay away from it. I have one computer that I use for downloading and nothing else. So when it gets to the point where I have so many things on it I just reformat and start over.
I have found limewire to work very well so far...