spamassasin of course.. .

• spamassassin.apache.org/
• http://apisnetworks.com/saconfig.php

also..

• http://www.kungfugrippe.com/previously/002462.php
• http://projects.puremagic.com/greylisting/
• baysean filtering
• poisoning the honey, etc+ (see also spamtrap )
• on the automatic writing of Spam Poetry

notes

• postfix, spamd and greylisting on openBSD » http://www.pantz.org/os/openbsd/postfix-spamd-dovecot.shtml / / etc+