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.

alexmoskovsky

Provided by <a href="http://multitoolbox.com/" rel="nofollow ugc">Multitoolbox</a>.
Simple, but strong captcha, built to prevent spam in WordPress comments. Succesfully blocks Xrumer-like tools.
To leave a comment on your blog, visitors need to simple check the captcha’s field. That’s all! Forget about annoying typeing of letters and digits.
Features:
<ul>
<li>Extremely simple passing for human;</li>
<li>Built-in id’s and function name’s generator to produce unique and random values, Xrumer won’t pass;</li>
<li>Catchy look, you commentators won’t miss;</li>
<li>Captcha’s code will be pasted into the comment form via JavaScript, so spam-bots will get a big parsing problems; </li>
<li>Comment text saving, so if commentators forgot to check captcha’s field, they never have to write commen again.</li>
</ul>
Languages:
<ul>
<li>English;</li>
<li>Русский;</li>
<li>Український (thanks to <a href="http://sweatego.ru/" rel="nofollow ugc">Sweat Tony</a>);</li>
<li>Italiano (thanks to <a href="http://sweatego.ru/" rel="nofollow ugc">Sweat Tony</a>).</li>
<li>Čeština (thanks to <a href="http://spa-ostrov.ru/" rel="nofollow ugc">Zeleny Rajce</a>).</li>
<li>Français (thanks to <a href="http://www.android-software.fr/" rel="nofollow ugc">Laurent</a>).</li>
<li>Беларуская (thanks to <a href="http://pc.de/" rel="nofollow ugc">Marcis G.</a>).</li>
</ul>
Amcaptcha is bulletproof solution tested on many blogs.

Amcaptcha

Simple, but strong captcha, built to prevent spam in Wordpress comments. One-click solution tested on many blogs!

Hiddy captcha 1.5.1 vs Amcaptcha