atitan

Math Quiz generates dynamic math problem in the comment form to beat spam robots and supports flexible form style customization.
Demo website: https://atifans.net/ (with Supercache and AJAX comment form enabled)
For PayPal donation, please use this email address: info a.t atifans.net
Features:
<ul>
<li>Dynamic problems prevent spam robots from posting unwanted content.</li>
<li>Quiz form inserted using AJAX for better compatibility with HTML cache plugins.</li>
<li>Form style and position can be customized in the control panel.</li>
<li>Trackbacks and Pingbacks are checked with DNS queries to avoid abuse.</li>
<li>Optional login page protection.</li>
</ul>
Translators:
<ul>
<li>SLV-es (Spanish)</li>
<li>Joao Paulo (Portuguese)</li>
</ul>

Math Quiz

Math Quiz generates dynamic math problem in the comment form to beat spam robots.

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.

Math Quiz vs Uber reCaptcha