Kasuga

Tiny Comment Spam Blocker is a lightweight yet powerful plugin designed to protect your WordPress comments from spam. It employs five different techniques to detect and block unwanted comments:
<ol>
<li>Nonce Verification – Ensures that the comment form submission is genuine.</li>
<li>Submission Time Check – Blocks comments submitted too quickly to prevent bots.</li>
<li>Honeypot Field – Hidden field that traps automated spam bots.</li>
<li>User Agent Validation – Detects suspicious User-Agent strings and blocks them.</li>
<li>Forbidden Word Filtering – Blocks submissions containing words or phrases from a configurable list within the comment body, email address, or IP address.</li>
</ol>
These filters are applied in order: if a comment passes the first check, it proceeds to the second, and so on, until all checks are applied or the comment is blocked.
Additional features:
<ul>
<li>Option to log detected spam in a local log file (up to 1.0 MB).</li>
<li>Optional email notifications when spam is detected.</li>
<li>Easy settings page in the WordPress admin panel.</li>
</ul>
<h3>Settings</h3>
The plugin provides the following settings in the WordPress admin panel:
<ul>
<li>Enable Spam Protection – Toggle the spam protection on or off. When disabled, all anti-spam checks are skipped.</li>
<li>Save Spam Detection Log – Enable or disable logging of detected spam. Logs are saved in a local file up to 1.0 MB within the WordPress uploads directory.</li>
<li>Notification Email Address – Enter an email address to receive notifications when spam is detected. Leave blank to disable email notifications.</li>
<li>Minimum Submission Time (seconds) – Set the minimum allowed time between loading the comment form and submitting a comment. Comments submitted faster than this threshold are considered spam.</li>
<li>Forbidden Words List – Enter one forbidden word, phrase, or IP address per line. Submissions containing these entries in the comment body, email address, or IP address will be blocked. Case is insensitive.
<ul>
<li>Example: 
viagra 
online pharmacy 
spam@email.com 
164.138.205.72</li>
</ul>
</li>
</ul>
<h3>Arbitrary Section</h3>
This plugin is designed to be lightweight and fast, ensuring minimal impact on site performance while providing robust protection against comment spam.

Tiny Comment Spam Blocker

A simple and lightweight yet rock-solid plugin that blocks comment spam using multiple automatic detection methods.

Denis V (Artprima)

AP HoneyPot WordPress Plugin, based on Jan Stępień’s http:BL, allows you 
to verify IP addresses of clients connecting to your blog against the Project 
Honey Pot database. Thanks to http:BL API you can quickly check whether your 
visitor is an email harvester, a comment spammer or any other malicious 
creature. Communication with verification server is done via DNS request 
mechanism, which makes the query and response even quicker. Now, thanks 
to AP HoneyPot WordPress Plugin any potentially harmful clients are denied 
from accessing your blog and therefore abusing it.
<h4>Your Feedback Matters</h4>
Bugs to report? Feature requests? Criticism? New ideas? We want to hear from 
you! Do not hesitate. Get in touch with us and share your views.

AP HoneyPot WordPress Plugin

Hard to believe I've been using this fro over 3 years, and the previous one before for even longer. No I wonder this is still being maintained, but still using in the meantime. 

Updated 3 years ago and still works great

AP HoneyPot WordPress Plugin allows you to verify IP addresses of clients connecting to your blog against the Project Honey Pot database.

Tiny Comment Spam Blocker vs AP HoneyPot WordPress Plugin