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.

Matt Keys

Anti-Spam solutions that require your users to fill out captcha’s are frustrating for actual humans, and not that great at stopping bots.
This solution works automatically without any user configuration, and puts no additional burden on your users. After activation this plugin will go to work stopping: comment spam, spam bot registration, and bbPress spam.
Key features:
<ul>
<li>Fully compatible with wordpress caching plugins or even more advanced server level caching solutions (like WP Engine uses).</li>
<li>Utilizes randomly generated and rotating token field name and token value, which stops more clever bots that might try caching the ‘solved’ field.</li>
<li>Anti-spam field is randomly placed within the form on page load to make it difficult for spammers to target the field.</li>
<li>Developer friendly with filters and actions</li>
<li>Lightweight, clean, and efficient solution to comment, registration, and bbPress spam</li>
<li>Uses native JavaScript to avoid jQuery dependancies</li>
</ul>
Requires JavaScript be enabled in client browser (Users will see a warning if JavaScript is disabled)

Advanced Invisible Anti-Spam

Block bots without annoying captchas. Cache friendly solution with rotating keys! Blocks comment, registration, and bbpress spam. Activate and done!

Hiddy captcha 1.5.1 vs Advanced Invisible Anti-Spam