joetan

A simple pre-filter to weed out the most obvious comment spam (about 90% of all spam).
Legitimate comments that get blocked (either by this plugin or by Akismet) can be presented with a captcha to confirm that the comment is legitimate. Comments that don’t pass the captcha will be immediately discarded.
Helps you identify potential spam words (you can use this to tweak the plugin’s filters).

TanTanNoodles Simple Spam Filter

A plugin that does a simple sanity check to stop really obvious comment spam before it is processed.

Sergey Biryukov

Checks via Google Search if the comment being left is a common spam comment.
Inspired by <a href="http://alexking.org/projects/wordpress" rel="nofollow ugc">Delink Comment Author</a> plugin from Alex King.

Unique Comments

Checks via Google Search if the comment being left is a common spam comment.

TanTanNoodles Simple Spam Filter vs Unique Comments