This plugin will add a ReCaptcha to all Zoo comment forms. You can also change the look of the ReCaptcha, just select one of four different themes. And it is also possible to log failed reCaptcha attempts and IP addresses (using the file specified by 'error_log' in php.ini).
Configuration is easy, just enable the plugin from Menu -> Extensions -> Plugin Manager and enter your public and private key that you received from ReCaptcha (see also use reCAPTCHA on your site)
You can see an example/demo http://www.yoonique.net/zoodemo/blog/2010/03/11/001/beautiful-iphone-wallpapers.html#comments