Stuart Sequeira

Block comment spam (up to 98% of it !) by inserting a question and answer field on your comment forms. No more wasting your time emptying out pages of spam messages just to get to the real comments from real visitors!
You can create your own simple question, like “What is 1 plus 1?” and the commenter must supply an answer on the comment form. If the commenter can’t answer the question correctly, this great little plugin blocks the comment from getting into your website.
Thanks to Andrew Kurtis and his team at Web Hosting Hub for translating this plugin into Spanish!
With the recently added blocked-spam counter, we collected data on our test site that blocked 798 spam comments and allowed only 14 through – a 98% success rate!

Comment Gatekeeper

Block spam comments by adding a simple question and answer field to your comment forms.

Automattic

Donncha O Caoimh (a11n)

This plugin adds a stylesheet or image to your blog’s html source code. When a browser loads that stylesheet or image a cookie is dropped. If that user then leaves a comment the cookie is checked. If it doesn’t exist the comment is marked as spam. 
The plugin can also check how long it took a user to enter a comment. If it’s too fast it’s probably a spam bot. How fast can a legitimate user enter their name, email, web address and enter a well thought out comment?
For the adventurous, add these lines to your .htaccess and it will block spam attempts before they ever get to WordPress. Replace the Xs with the cookie that was set in your browser after viewing your blog. You can also find the cookie value by examining the page source code and looking for “css.php?k=XXXXXXXXXXXXXXXXXXX”. Make sure the lines go above the standard WordPress rules.
<pre><code> RewriteCond %{HTTP_COOKIE} !^.*XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.*$
 RewriteRule ^wp-comments-post.php - [F,L]
</code></pre>
If you use WordPress MU, replace wp-comments-post.php above with wp-signup.php to block spam signups.
<pre><code> RewriteCond %{HTTP_COOKIE} !^.*XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.*$
 RewriteRule ^wp-signup.php - [F,L]
</code></pre>

Cookies for Comments

If you don't use any other spam filter plugins, then you can rely on this to stop spam comments for sure!

Must have, if you don't use any other spam filters!

I'd been getting a dozen spams a day on two of my sites. We installed "Stop Spam Comments" on one and "Cookies for Comments" on the other. Both worked perfectly. No more spam. Thanks!



very easy and no more spam!

Sets a cookie on a random URL that is then checked when a comment is posted. If the cookie is missing the comment is marked as spam.

Comment Gatekeeper vs Cookies for Comments