er11

e11 reCAPTCHA is a no-frills way to add Google’s reCAPTCHA widget to the standard WordPress comment form and/or the user registration page. By default it will require only logged-out users to solve CAPTCHAs on comments and new users to solve a CAPTCHA to successfully register on the site; it’s also possible to configure e11 reCAPTCHA to not require CAPTCHA solving in either of these two forms or to require it on comments by all users.

e11 reCAPTCHA

WordPress plugin to use reCAPTCHA to help protect comments and new user signups against spam.

Silkalns

WP Chill

Cristian Raiber

A very useful plugin for everyone using WordPress. Adds reCaptcha security to the WordPress login form, register form and comment form. This plugin could help your blog get rid of a lot of spam comments or brute-force attacks.
Nothing gets passed it if the reCaptcha doesn’t validate.
A few notes about the plugin:
<ul>
<li>Supports audio or image captcha types</li>
<li>Can generate the reCaptcha image / audio type in a number of predefined languages</li>
<li>Adds reCaptcha protection to the WordPress login form</li>
<li>Adds reCaptcha protection to the WordPress register form</li>
<li>Adds reCaptcha protection to the WordPress comment form</li>
<li>Adds reCaptcha protection to the WordPress recover password form</li>
</ul>

Uber reCaptcha

This plugin adds the reCaptcha form to the WordPress login form, recover password form, register form and comment form.

e11 reCAPTCHA vs Uber reCaptcha