Announcing Release Candidate 1 of Bad Behavior.
Bad Behavior is a set of PHP scripts which prevents spambots from accessing your site by analyzing their actual HTTP requests and comparing them to profiles from known spambots. It goes far beyond User-Agent and Referer, however.
Spam Karma and similar solutions should be your last line of defense against comment spammers, not your first. If the spambot has fetched a page from your site, you have already lost: the spammer now has your email addresses and your comment form, you are paying for the wasted bandwidth, and you can bet he'll be back. Bad Behavior is your first line of defense.
In pre-release testing Bad Behavior blocked spambots responsible for comment spam, referrer spam and email spam, leaving virtually nothing for Referrer Karma and Spam Karma to do. In fact, we disabled Referrer Karma, and Bad Behavior promptly picked up the slack and blocked all the referrer spam coming in.
Bad Behavior not only can function as a WordPress plugin, it can also be integrated through a "generic" interface to virtually any PHP-based software.