I've been waiting forever for someone tp pick up doing a particular project to fight comment spam. A couple of days ago I got tired of waiting and put it together in an evening. It's no silver bullet, but it did cut the amount of comment spam I get down to a fraction of what it was before. And it should scale in a way so the spammers can't easily program their way around it if many people start using it.
No, doesn't require any extra work for your users or any template changes. It's running on this weblog already.
If you either run MT on your weblog or have some time to nudge your favorite weblog software and would like to help me test it, send me a mail.