ok Kazaa, the "Old" morpheus and Grokster are all on the FastTrack P2P network, which is now owned by an Aussie company who want everyone who uses their tech to use spyware,
now Morpheus trumpeted the idea it would never have spyware, so it ditched the Fast Track network in favour of the Gnutella P2P network, to do so quickly it grabbed the source from Gnucleus (which is free btw with no ads) and put their ad system into it
the FastTrack network works better IMO but it requires you to log on to a server at start for the ads, thus some smart peeps are doing their own open version with no server and no ads,
Gnutella is nice on a small scale but it can have a hard time at a large scale, although bearshare and Limewire are working to get around those problems
for the future though I recomend the giFT network, orginally a hack of the FastTrack network, they gave up since the FastTrack people kept changing encryption algorithms thus, they decided to move to open source, currently though there isn't a windows port...
check it out here
http://gift.sourceforge.net/
and a short little article about it here:
http://www.cdfreaks.com/news2.php3?ID=3591