digitalzoomstudio

This plugin creates a capcha box before your submit button in the comments section to prevent all that unwanted spam. It comes with a mini-admin panel and love.
UPDATED 1.1 
– fixed bugs 
– no more manual input of the submit button
LIVE DEMO – http://digitalzoomstudio.net/installing-magento-on-mamp/221
TWITTER – http://twitter.com/ZoomItFlash 
FACEBOOK – http://www.facebook.com/digitalzoomstudio
Check my other premium wordpress plugins here – http://bit.ly/dOenET

Comments Capcha Box

This plugin creates a capcha box before your submit button in the comments section to prevent all that unwanted 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.

Comments Capcha Box vs Uber reCaptcha