MikesPickz

MP Spam Be Gone is the simplest, most effective Spam blocker without difficult CAPTCHA responses needed by the user. This plugin was inspired by some of the best Anti-Spam plugins available on WordPress.
Inspirations: 
Spam Free WordPress by Todd Lahman, LLC 
Growmap Anti Spambot Plugin by Andy Bailey 
Simple Trackback Validation with Topsy Blocker by Tobias Koelligan
This plugin has aspects from all of the above mentioned plugins to form the most complete and dead simple Anti Spam plugin on WordPress. This plugin doesn’t write anything to your database while preventing comment spam and trackback spam.
From Growmap Anti Spambot Plugin, I used the concept of having a checkbox display to the user in order to prevent automated Spam. To me this is the best way (and least intrusive) to completely prevent automated spam from ever even being written to the database in the spam que. It is physically impossible to get a false positive, since you either checked the button or not. The flaw with this plugin is that it does not work with Javascript disabled.
To combat that flaw, I use the copy and paste password fields from the Spam Free WordPress plugin. These fields are in a <code><noscript></code> tag, and only appear if Javascript is disabled. Unlike, the Spam Free WordPress plugin, these passwords are generated on the fly and never stored in the database.
These plugins primarily stop comment spam. In order to stop trackback spam, I added the IP address check, URL check, and Topsy check functions from Simple Trackback Validation with Topsy Blocker. If a trackback is identified as spam because of the IP address or Topsy, it is deleted and never written to the database. If a trackback is identified as spam because of the URL, it is marked as spam and sent to the Spam que. The reason that these are not automatically deleted, is because it is possible that the referring page is using a shortlink and therefore the full URL would not appear on the page in the first place.
When combined, this plugin will stop all automated spam and trackbacks.
It is intended to replace the above plugins.
You can’t find a plugin simpler than this – set it and forget it. No options = no modifications to the database.

MP Spam Be Gone

MP Spam Be Gone is the simplest, most effective Spam blocker.

OneOfOne

This plugin plugin blocks spam in comments automatically, without requiring any end-user input or any javascript.
<ul>
<li>As of version 0.7.7 there’s an option to add an extra javascript check, it’s off by default.</li>
</ul>
In a way it’s similar to webvitaly’s <a href="https://wordpress.org/plugins/anti-spam/" rel="ugc">Anti-Spam</a> however this doesn’t require the user to enter anything at all.
The comment gets marked as spam if any of the following rules are true :
<ul>
<li>If the comment is a trackback.</li>
<li>If the time between loading the page and commenting is less than 10 seconds.</li>
<li>If the Session variable specific to this form is not set.</li>
<li>If the hidden input field have a different value than “-“.</li>
<li>If the comment includes more than 3 urls.</li>
<li>If the referer isn’t set properly.</li>
</ul>
Once the comment gets flagged as spam, and if the auto delete option isn’t set, a json string will be appended to it to show why it was marked, for example :
<pre><code>{ "is-trackback": 0, "no-session-token": 0, "hidden-field": 1, "number-of-urls": 5, "referer": 0, "too-fast": 1.902538061142 } </code></pre>
Translates to :
<ol>
<li>They changed the hidden input field.</li>
<li>They Had 5 URLs in the comment.</li>
<li>It Took 1.9 seconds to submit the comment since the page was loaded.</li>
</ol>
Also note that the time calculations are per-form, so there are no false-positives if the user has multiple pages open on the site and commented on 2 of them in a short period of time.
Feel free to fork it and submit patches / fixes on <a href="https://github.com/OneOfOne/ooo-nospam" rel="nofollow ugc">github</a>

OneOfOne’s NoSpam

Extremely simple, 0 configuration and 0 user/admin-interaction anti-spam plugin.

MP Spam Be Gone vs OneOfOne’s NoSpam