Google Captcha plugin for WordPress by bestwebsoft allows you to implement a super security captcha form into web forms. Google Captcha is a free CAPTCHA service that helps to digitize books, newspapers and old time radio shows. This captcha can be used for login, registration, password recovery, comments forms.
Features
Actions: You can add Google Captcha to any standard form – login, comments etc.
Actions: You can choose the users, for whom captcha will be hidden.
Display: You can choose one of four standard Google Captcha themes.
Download: http://wordpress.org/plugins/google-captcha/