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.

IceCaptcha

The new generation “IceCaptcha” plugin.
The new security system presupposes clicking on the circle of a particular color instead of entering numbers or other redundant actions. It is done instead of clicking on the button, i.e. no spare action is required!
As for the bots or foreign captcha entering services, the task practically can’t be solved as the circle is drawn randomly in various places, it is of random color and the background (the picture itself) is also different every time. The click spot coordinates are encrypted and sent to the server where the checkup is performed. This all brings us to the fact that there is no chance of switching or calculating the correct result for any third party software.
You can use our captcha for: • Registering new users • Authenticating users • Commenting anti spam security

This is the new generation IceCaptcha plugin. The Anti-Spam security system misses no bots and does not disturb a human being.

Hiddy captcha 1.5.1 vs IceCaptcha