I can't believe no one has mentioned BearShare.
Absolutely the best P2P program I've ever used, ever. It operates on the same Gnutella network as LimeWire and Grokster, etc, so it's able to retrieve not only results of its own, but results that Limewire and Grokster would also find.
Honestly, with a Cable/T1 connection that I have, I'm able to download songs (roughly 5MB each) in less than a minute.
