OpenBSD spamd - greylisting and beyond is a presentation by Bob Beck of the University of Alberta about the OpenBSD's spamd system, how spammers react to it and similar systems, how you can exploit this, and the University of Alberta's experiences with spamd, complete with interesting numbers. I'm sadly jealous, as local feelings insure that I'm not going to get to deploy this sort of technology any time soon.

(Also from Richard Johnson of river.com.)