dannyx

This is my first WordPress plugin that i made it for personal use and to learn how to build plugins. 
This plugin replaces the submit button on the post comment section with a set of buttons from images. 
A small message provides the user a hint on what button to use to submit the comment. 
Demo: http://www.arcweb.ro/blog/2011/12/21/testing-cutecaptcha-plugin/
Requirements: 
PHP mcrypt should be available on the server, i will add more encryption methods if required. 
Session support is optional, supports caching of post pages without session but with lower security.
Customize: 
All the information and parameters to configure are in cutecaptcha.php 
The images and messages can be customized and should be quite fun to adapt it with the 
subject of each website, especially expecting that anybody that wants to post a comment 
should have a good idea about that subject.
Security: 
The variety in images and messages will make it difficult for automated bots to break it. 
The plugin is intended for a regular blog that will never attract dedicated spamers. 
At this moment the association between messages and photos can be analyzed, but it is possible to add 
another layer of protection if required. I strongly suggest that each site uses personalized images and 
messages related to their content.
TODO: 
Alternative encryption functions for servers without php mcrypt. 
Extra layer of security on images: Raw data inserted in file, modified with 
variable watermarks for each session. 
Display a specific number of buttons from a gallery of buttons. 
IP Block: block ip’s after a number of failed submits for a number of days.

Cute Captcha

Replaces the submit button  with a set of buttons from a set of images. A small message provides the user a hint on what button to use.

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.

Cute Captcha vs Uber reCaptcha