Six Apart Guide to Combatting Comment Spam

Here is a nice guide to comment spam and how to fight it, provided by Six Apart, the makers of Movable Type.

Six Apart Guide to Combatting Comment Spam:

This document describes how malicious or unwanted comments (‘comment spam’) affect weblogs, the techniques spammers use to abuse weblogs, and the tactics that can be used to prevent and defend against these attacks. Also included is a review of the strengths and weaknesses of each tactic, instructions for implementing them on your weblog and ones which we recommend for the best protection.