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.

webjema

Adds CAPTCHA anti-spam methods to WordPress on the comment form, registration form, or both. 
In order to post comments, users will have to range icons in right (upright) place. 
This can help prevent spam from automated bots (100%). This will also increase the quantity of comments because of funny icons.
Requirements/Restrictions:
<ul>
<li>Works with WordPress 2.x, 3.x.</li>
<li>PHP 4.3.0 or above with GD2 library support.</li>
<li>Your theme must have a <code><?php do_action('comment_form', $post->ID); ?></code> tag inside your comments.php form. Most themes do. 
The best place to locate the tag is before the comment textarea, you may want to move it if it is below the comment textarea.</li>
</ul>
Features:
<ul>
<li>Configure from Admin panel</li>
<li>JavaScript is not required (but desirable)</li>
<li>Allows Trackbacks and Pingbacks</li>
<li>Setting to hide the CAPTCHA from logged in users and or admins</li>
<li>Setting to show the CAPTCHA on the comment form, registration form, or both</li>
<li>I18n language translation support (see FAQ)</li>
</ul>
<h3>Configuration</h3>
After the plugin is activated, you can configure it by selecting the <code>NotCaptcha options</code> tab on the <code>Admin Plugins</code> page. 
Here is a list of the options:
<ol>
<li>
NotCaptcha on Register Form: – Enable captcha on the register form.
</li>
<li>
NotCaptcha on Comment Form: – Enable captcha on the comment form.
</li>
<li>
NotCaptcha on Comment Form: – Hide captcha for registered users (select permission level)
</li>
<li>
Noise on captcha: – Add some shapes to protect the image from auto-recognition
</li>
<li>
Background color of images: – Set color similar to your blog background
</li>
</ol>
<h3>Usage</h3>
Once activated, a captcha section is added to the comment and register forms.
<h3>Version History</h3>
rel 1.3.1 (30 October 2011)
<ul>
<li>Added function “imagerotateEquivalent” (works if your php installation do not has “imagerotate” function)</li>
<li>Gallery folder disabled from listing</li>
<li>Added new noise to images</li>
<li>Added new icons to gallery </li>
<li>Added Spanish language</li>
</ul>
rel 1.3 (27 June 2011)
<ul>
<li>Added mobile devices support (rotate by clicking on image)</li>
<li>Code optimization (no captcha form – no css and js in page header)</li>
<li>Added new icons to gallery</li>
</ul>
rel 1.2 (3 May 2010)
<ul>
<li>The method to check the correct answer has changed</li>
<li>Corrected problem if blog installed in subfolder</li>
<li>Added “Reload images” button</li>
<li>CSS problems corrected</li>
<li>Added “noise” option</li>
<li>Added “background color” option</li>
<li>Added new icons to gallery</li>
<li>A lot of small corrections</li>
</ul>
rel 1.1 (3 Jul 2009)
<ul>
<li>Initial Release</li>
</ul>

WP-NOTCAPTCHA

New kind of human test. Adds CAPTCHA anti-spam method to WordPress on the comment form, registration form, or both.

Hiddy captcha 1.5.1 vs WP-NOTCAPTCHA