Here is my latest contribution to fight spam:
http://www.onlamp.com/pub/a/bsd/2007/01/18/greylisting-with-pf.html
Greylisting
About The Author
Dan Langille
I've been playing with computers since I read an Elementary Electronics magazine way back in the 1970s. I started contributing to open source projects in 1998. After that, I gradually moved from being a software developer to being a systems administrator.