Cute Captcha is a WordPress plugin by dannyx that replaces the submit button with a set of buttons from a set of images. A small message provides the user a hint on what button to use.
Requirements: PHP mcrypt should be available on the server
Demo: http://www.arcweb.ro/blog/2011/12/21/testing-cutecaptcha-plugin/
Download and info: http://wordpress.org/extend/plugins/cutecaptcha/