Eric's Archived Thoughts: WP-Gatekeeper is a very interesting approach to comment spam: it simply asks one of many pre-configured questions that a human can answer very easily, but a spam bot cannot. Similar forms are already being used in various blogs, but here it's nicely worked out (although in my view it could also be completely realized as a plugin). The basic idea is essentially that of a CAPTCHA - but a textual CAPTCHA. A human can easily answer the question what is 1+1 - a spam bot won't get anywhere with that. Sure, spammers can create databases of questions and answers. But if everyone sets up their own collection of questions, it won't get them far. For comment spam, it should be a very usable solution.
Unfortunately, there's no such simple solution for trackbacks...
Update: since I find the idea somewhat amusing, I'm currently writing a corresponding plugin. So it's possible that my comments might behave a bit strangely tonight.