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. Bad Behavior is available for several PHP-based software packages, and also can be integrated in seconds into any PHP script.
Newsfeed[edit]
<rss>http://error.wordpress.com/tag/bad-behavior/feed/</rss>