sdesalas

This plugin allows WordPress to block over 95% of spam comments using a time-based hash.
This allows for a minimum sanity check and yet should remove almost all spam comments 
without the need to sign up to any third party APIs.
The plugin has been tested in real world conditions for around 12 months. It will not block 
real comments and it is pretty effective against spam, however it only uses a very simple 
algorithm and for that reason some spam comments may still make it through.

WP Simple SpamCheck

This plugin allows WordPress to block over 95% of spam comments using a time-based hash.

web2dev

A simple comment captcha protection based on <a href="http://captcha.hsoub.com/" rel="nofollow ugc">Hsoub API</a> (support arabic & english).
<h3>About the plugin</h3>
This plugin use the great <a href="http://captcha.hsoub.com/" rel="nofollow ugc">Hsoub API</a>, and is a <a href="http://web2dev.me/" rel="nofollow ugc">web2dev</a> concept.

WP Simple SpamCheck vs Hsoub CAPTCHA