http://www.spamcop.net/
(Note the .net appendage, and accept no substitutes!)
This site processes spam emails and finds out who to complain to. Once it does that, it's a matter of point-and-click to send out the complaints.
Referrers: http://www.modemac.com/cgi-bin/wiki.pl/The_War_of_the_Worlds_(1953)/Cast_Iron_Chaos/Know_Your_Enemy/Net_Scum_Page/RecentChanges/Cast_Iron_Chaos/Food_and_Cooking/Adventures_in_Culimancy/Eggnog_Cake/Cast_Iron_Chaos/Scumbags_Spammers_and_Scammers