I would love it if someone would port over the NoSpam! mod
from vBulletin. It's an alternative to CAPTCHA that allows you to set questions and answers to be randomly asked during registration, and if the given answer doesn't match registration is denied. Even something as simple as "Type the word 'blue'" or "What animal is this?" while displaying an image of a cat will work. It has stopped spam registrations dead in their tracks on the one vBulletin forum I have.