Below I have collected to links to information / sites about greylisting / anti-spam. If you have any suggestions for new links please contact me (contact info on the about page).
Nolisting.org: Poor Man's Greylisting
LWN: Spam blocking with greylisting
Slashdot | The Next Step in Fighting Spam: Greylisting
AutumnTECH: products and services to protect companies from spam/virusses