Captcha Bank is a anti-spam captcha plugin for WordPress forms that adds secure image captcha or math captcha to prevent spam.

CH Captcha use for Add captcha to WordPress on contact form 7.

Stop spam bots from registering, logging in, filling out contact forms and commenting on your site without using annoying Captcha codes or puzzles.

WP reCaptcha authenticate the visitor is a human verifying by Google ReCaptcha