One method that is worth mentioning is tarpitting. Most anti-spam methods stop the spam from getting the users attention by either rejecting it at SMTP time or filtering it later. Tarpitting takes the fight to the spammers by wasting their mailers time. The ultimate aim is that there will one day be enough tarpits in the world to make spamming impossible.

Check out the excellent work by the spamcannibal project at http://spamcannibal.org/cannibal.cgi they have a functional SMTP tarpit for linux and a blacklist that is kept up to date.

Personally I don't use their tarpitting software, But I do use their blacklist with OpenBSD's spam tarpit. My primary MX is on OpenBSD.