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/30_Minutes_With_the_Lights_Off_(in_Color)/Cast_Iron_Chaos/Books_and_Zines/The_Chap/Cast_Iron_Chaos/Scumbags_Spammers_and_Scammers