Six Apart Guide to Comment Spam
Six Apart Guide to Comment Spam explains how comment spam works and how to fight it. Parts of the document are specific to MovableType, but it should still be interesting to users of other publishing systems.
I have implemented several of the techniques mentioned in the guide, and currently all automated spam gets caught. There is still the occasional manual spam that gets through. Nothing stays online for long though, so spammers need to be really lucky for Google to crawl the site before their junk gets deleted.