kazaa, full of sh*te, viruses, spyware.
areslite, plenty of sharers, massive data base, slow downloads (200kbps), viruses, spyware though easily picked up with any software.
limewire, fastest download times, viruses slip through loads of AV software, not many sharers, sh*te downloads, downloads are miss perceived i.e download superman get hitchhikers guide to the galaxy kinda thing, you get this with all but especially more with limewire.
these are all free p2p, if you pay for it, roughly 12-15 quid a year its a different kettle of fish, still SOME malware and a lot quicker to download, hope these help.
though for films and music especially albums and compilations i would use this
http://www.mininova.org/cat/