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.

latcomsystems

By default, all images in WordPress posts link to themselves. When a user sees a link, they click on it, and the image shows up by itself for no apparent reason. This has several drawbacks:
<ul>
<li>This is bad for SEO because these links are dead ends.</li>
<li>User experience is diminished due to wasted clicks.</li>
<li>Potential for higher bounce rate since no easy method is provided to get back to the site except for the browser “Back” button.</li>
</ul>
This plugin uses a standards compliant HTML DOM parser (http://sourceforge.net/projects/simplehtmldom/). It does not use regular expressions (regex) to accomplish link removal and therefore will not break your site pages or mess up your theme.
<h3>Support</h3>
<ul>
<li><a href="mailto:sysdev@latcomsystems.com" rel="nofollow ugc">sysdev@latcomsystems.com</a></li>
</ul>

LCS Image Nolink

New images inserted into posts will have no links by default.  Existing self-links on all images are removed before the post content is shown.

WP Simple SpamCheck vs LCS Image Nolink