Harvesting addresses from P2P
Many users of P2P networks (Kazaa, Gnutella etc) unwittingly share their whole hard drives. In addition to confidential information, you’d then be exposing your friends’ e-mail addresses to spammers. Because they do look for those kinds of files and download them, resulting in a shitload of spam for those on your list.
Be careful with the last paragraph on the page there. Any kind of opt-out registry can be misused by spammers, unless certain requirements are met.
Via Anti spam blog