Ryan McCue

<p>Many spammers bypass the comment form and send requests to the WordPress backend directly. This plugin creates a “nonce” field for your comment form, similar to the ones used in the administration backend.</p>
<h3>Troubleshooting</h3>
<p>RSCV not working? Use the following steps to try and solve the problem.</p>
<ol>
<li>Make sure <code><?php do_action('comment_form'); ?></code> is in your template’s comments.php, inside the <code><form></code> tags.</li>
<li>Still not working? Ask on the forums.</li>
</ol>


Really Simple Comment Validation

Adds a nonce field to the comment form to deter some comment spammers who bypass the comment form.

Silkalns

WP Chill

Cristian Raiber

<p>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.</p>
<p>Nothing gets passed it if the reCaptcha doesn’t validate.</p>
<p>A few notes about the plugin:</p>
<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.

Really Simple Comment Validation vs Uber reCaptcha