Amanda &amp; Steve

Is this the ultimate in automated ‘bot Spam Killers?
Spam is the bane of WordPress admin’s lives; auto-generated Spam can bypass unset hidden fields from the WordPress comment page by directly submitting the comment to WordPress.
This plugin does two things:
<ul>
<li>Removes the ‘Comment Author URL’ field from the WordPress comments form; for most non-technical blogs this optional field is irrelevant.</li>
<li>Kills WP (hey that saves some CPU cycles) if a comment is submitted which contains a Comment Author URL.</li>
</ul>
Website spam ‘bots have two aims, firstly they populate spam links within the main body of a spam comment, and the spam ‘bot also includes the spam destination in the Comment Author URL of the comment field.
We put together this little plugin as despite running two other anti-spam plugins + Google Captcha, ‘bot spam was still getting through. This was resulting in an hour or so each week weeding out the real comments that had fallen into the spam bucket due to the real commenter failing to complete the Google Captcha correctly. We also noticed that real user comments reduced with Google Captcha in place.
We don’t need to stand for this any longer; say goodbye to Comment Author URL Spam and save CPU cycles!

TMD Spam Killer

Hides the 'Comment Website URL' field, kills WP if a spam 'bot submits a comment which includes a Comment Author URL.

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.

TMD Spam Killer vs Uber reCaptcha