This plugin provides you an easy captcha without javascript, without images, without math. Only a few of CSS and a few of server side. 
An hidden text input is placed in comment form a human cannot see it so a human cannot compile it, a spambot otherwise see the text input and is forced to compile it. 
The plugin check the existence of this text input and check if it has a value or not. It drops all comment with a value in the text input as spam. 
Really simple and powerfull antispam system. Low CPU usage and no access to DB.

Hiddy captcha 1.5.1

This plugin provides you an easy captcha without javascript, without images, without math. Only a few of CSS and a few of server side.

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.

Hiddy captcha 1.5.1 vs Uber reCaptcha