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.

Daniel Davis

No-captcha Spam Block is a WordPress plugin that automatically blocks comments it thinks are spam. It works tirelessly, day and night, applying various checks to each comment that comes in and giving it a spam score – any score too high and the comment goes straight to the spam bucket in disgrace.
<ul>
<li>No third-party registration</li>
<li>No JavaScript required</li>
<li>Best of all, no captchas</li>
</ul>
There are no visible changes to your website and you don’t need to set anything up – just install, activate and go!

No-captcha Spam Block

Dramatically reduce comment spam on your blog without using a captcha.

Hiddy captcha 1.5.1 vs No-captcha Spam Block