Stuart Sequeira

Block comment spam (up to 98% of it !) by inserting a question and answer field on your comment forms. No more wasting your time emptying out pages of spam messages just to get to the real comments from real visitors!
You can create your own simple question, like “What is 1 plus 1?” and the commenter must supply an answer on the comment form. If the commenter can’t answer the question correctly, this great little plugin blocks the comment from getting into your website.
Thanks to Andrew Kurtis and his team at Web Hosting Hub for translating this plugin into Spanish!
With the recently added blocked-spam counter, we collected data on our test site that blocked 798 spam comments and allowed only 14 through – a 98% success rate!

Comment Gatekeeper

Block spam comments by adding a simple question and answer field to your comment forms.

WP Chill

Silkalns

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.

Comment Gatekeeper vs Uber reCaptcha