If you are using WordPress and you are not using Askimet, you are missing out on some really sweet spam control.
Askimet works well because it’s a centralized database of spam that learns from every blog that uses it. Since blog spammers generally post the same message to a zillion different blogs, and since many of those blogs are using WordPress and Askimet, chances are the spam will be caught before it ever gets to you, or your blog.
I have been using it for a while now and it has caught all but a few spam messages. It’s almost effective enough to consider turning off manual comment approval without relying on some annoying CAPTCHA solution. If comment volume increases I will consider turning off comment review.