D'Arcy Norman

A very simple mu-plugin to automatically (automattically?) insert a “Spam prevention powered by Akismet” notice (with hyperlink to Akismet.com) in the footer of each blog hosted on a WordPress Multiuser service to abide by the licensing terms of Akismet. Adds a small “Spam prevention powered by Akismet” to the footer of every blog to abide by Akismet licensing terms.

Akismet Credit Inserter

Adds a small "Spam prevention powered by Akismet" to the footer of every blog to abide by Akismet licensing terms.

Brandon Fenning

Since the vast majority of WordPress spam is automated in nature, the goal of BotBlocker is to block comment spam bots and not hinder users. This is accomplished through the use of honeypot technology, which tricks the spambot into identifying itself by performing actions that a human could not.
BotBlocker’s features include:
<ol>
<li>Works out of the box to block spam</li>
<li>Automatically stops spam-bots from commenting</li>
<li>Zero hindrance to users: no CAPTCHAS, math questions, passwords or extra input required</li>
<li>Zero false positives and ignores registered users</li>
<li>Can be configured to completely block spam-bots or flag comments as spam</li>
<li>Spam detection messages can be easily adjusted</li>
<li>No javascript or cookies required</li>
<li>Hooks into <code>wp-comments-post.php</code> to prevent spam-bots from directly submitting spam comments</li>
<li>Should work fine with most customized comment forms</li>
<li>No API keys required</li>
</ol>
http://www.lform.com/botblocker/ for more information.
Plugin by <a href="http://www.lform.com/" rel="nofollow ugc">LFORM, a web design company</a> in New Jersey.

BotBlocker

Kills spam-bots, leaves humans standing. No CAPTCHAS, no math questions, no passwords, just spam blocking that stops spam-bots dead in their tracks.

Akismet Credit Inserter vs BotBlocker