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.

Noname

!!! IMPORTANT !!! 
If you have used the old Geo-Captcha plugin please completly uninstall it (the form is on the main page of the plugin) 
and remove the plugins files. Geo-Captcha & Geo-Blacklist ist fully based on the old Geo-Captcha plugin but is not 
compatible with it and uses new names for options and databases! 
!!! IMPORTANT !!!
There are a lot of ways to prevent spam. One that is still pretty often used is to let your readers type a code from a 
captcha image to a textbox. Geo-Captcha uses the same technic but you can define a whitelist with countries. People from 
this countries will not be forced to enter the captcha code. You can also define a blacklist which prevents people from 
some countries from posting any comments.
The location of the visitors is checked using the GeoLite Country-Library from MaxMind (http://www.maxmind.com). 
The captcha image is generated using SecurImage from drew010 (http://www.phpcaptcha.org).
<ul>
<li><a href="http://plugins.deformed-design.de/" rel="nofollow ugc">Plugin Page</a></li>
<li><a href="http://support.deformed-design.de/" rel="nofollow ugc">Plugin Support</a></li>
</ul>

Geo-Captcha & Geo-Blacklist

Geo-Captcha shows a captcha image only to countries you don't trust. Geo-Blacklists allows you to disable comments for some countries.

Hiddy captcha 1.5.1 vs Geo-Captcha & Geo-Blacklist