webjema

Adds CAPTCHA anti-spam methods to WordPress on the comment form, registration form, or both. 
In order to post comments, users will have to range icons in right (upright) place. 
This can help prevent spam from automated bots (100%). This will also increase the quantity of comments because of funny icons.
Requirements/Restrictions:
<ul>
<li>Works with WordPress 2.x, 3.x.</li>
<li>PHP 4.3.0 or above with GD2 library support.</li>
<li>Your theme must have a <code><?php do_action('comment_form', $post->ID); ?></code> tag inside your comments.php form. Most themes do. 
The best place to locate the tag is before the comment textarea, you may want to move it if it is below the comment textarea.</li>
</ul>
Features:
<ul>
<li>Configure from Admin panel</li>
<li>JavaScript is not required (but desirable)</li>
<li>Allows Trackbacks and Pingbacks</li>
<li>Setting to hide the CAPTCHA from logged in users and or admins</li>
<li>Setting to show the CAPTCHA on the comment form, registration form, or both</li>
<li>I18n language translation support (see FAQ)</li>
</ul>
<h3>Configuration</h3>
After the plugin is activated, you can configure it by selecting the <code>NotCaptcha options</code> tab on the <code>Admin Plugins</code> page. 
Here is a list of the options:
<ol>
<li>
NotCaptcha on Register Form: – Enable captcha on the register form.
</li>
<li>
NotCaptcha on Comment Form: – Enable captcha on the comment form.
</li>
<li>
NotCaptcha on Comment Form: – Hide captcha for registered users (select permission level)
</li>
<li>
Noise on captcha: – Add some shapes to protect the image from auto-recognition
</li>
<li>
Background color of images: – Set color similar to your blog background
</li>
</ol>
<h3>Usage</h3>
Once activated, a captcha section is added to the comment and register forms.
<h3>Version History</h3>
rel 1.3.1 (30 October 2011)
<ul>
<li>Added function “imagerotateEquivalent” (works if your php installation do not has “imagerotate” function)</li>
<li>Gallery folder disabled from listing</li>
<li>Added new noise to images</li>
<li>Added new icons to gallery </li>
<li>Added Spanish language</li>
</ul>
rel 1.3 (27 June 2011)
<ul>
<li>Added mobile devices support (rotate by clicking on image)</li>
<li>Code optimization (no captcha form – no css and js in page header)</li>
<li>Added new icons to gallery</li>
</ul>
rel 1.2 (3 May 2010)
<ul>
<li>The method to check the correct answer has changed</li>
<li>Corrected problem if blog installed in subfolder</li>
<li>Added “Reload images” button</li>
<li>CSS problems corrected</li>
<li>Added “noise” option</li>
<li>Added “background color” option</li>
<li>Added new icons to gallery</li>
<li>A lot of small corrections</li>
</ul>
rel 1.1 (3 Jul 2009)
<ul>
<li>Initial Release</li>
</ul>

WP-NOTCAPTCHA

New kind of human test. Adds CAPTCHA anti-spam method to WordPress on the comment form, registration form, or both.

Samir Shah

This plugin has two modes – basic and Akismet-enhanced.
Basic mode: the plugin will serve a CAPTCHA to all commenters that aren’t logged in and don’t have a previously approved comment. Repeat commenters will never see a CAPTCHA.
Akismet-enhanced mode (recommended): the plugin will serve a CAPTCHA only if Akismet identifies a comment as spam (much less frequently than the basic mode). Just install an activate <a href="https://wordpress.org/extend/plugins/akismet/" rel="ugc">Akismet</a> to enable this mode.
Note that a CAPTCHA will only appear after a comment is submitted. When a commenter is served a CAPTCHA:
<ul>
<li>If they fail, then the comment will be automatically discarded or trashed (and won’t clutter up your spam queue).</li>
<li>If they pass, it will be allowed into the spam queue (or approved, if you so choose). This means that any false positives from Akismet will be easily identified without you having to trawl through all the spam comments manually.</li>
</ul>
Most genuine commenters will be able to comment on your site hassle-free, without ever seeing a CAPTCHA.
The default CAPTCHA is a simple text-based test. There is also the option to use <a href="http://www.google.com/recaptcha" rel="nofollow ugc">reCAPTCHA</a> if you want something more robust (free API key required). You can also style the CAPTCHA page to fit with your own WordPress theme.
If you come across any bugs or have suggestions, please use the plugin support forum or <a href="http://rayofsolaris.net/contact/" rel="nofollow ugc">email me</a>. I can’t fix it if I don’t know it’s broken! Please check the <a href="https://wordpress.org/extend/plugins/wp-conditional-captcha/faq/" rel="ugc">FAQs</a> for common issues.
Thanks to the following people for contributing translations of this plugin: Belorussian – <a href="http://pc.de" rel="nofollow ugc">Marcis G</a>, Brazilian Portuguese – Stefano, Czech – <a href="http://trumplin.com/" rel="nofollow ugc">Ted</a>, Danish – Jesper, Dutch – <a href="http://wpwebshop.com/books/" rel="nofollow ugc">Rene</a>, Estonian – <a href="http://www.itransition.com" rel="nofollow ugc">Itransition</a>, Finnish – Jani, French – <a href="http://android-software.fr" rel="nofollow ugc">Laurent</a>, German – <a href="http://jochenpreusche.com" rel="nofollow ugc">Jochen</a>, Hindi – <a href="http://outshinesolutions.com" rel="nofollow ugc">Outshine Solutions</a>, Hungarian – <a href="http://www.televizio.sk" rel="nofollow ugc">Gyula</a>, Italian – <a href="http://gidibao.net" rel="nofollow ugc">Gianni</a>, Lithuanian – Mantas, Polish – <a href="http://www.spin.siedlce.pl" rel="nofollow ugc">Pawel</a>, Romanian – <a href="http://simplu.mixnet.ro" rel="nofollow ugc">Lotus</a>, Russian – <a href="http://www.everycloudtech.com/" rel="nofollow ugc">Ivanka</a>, Spanish – <a href="http://www.reinventia.net" rel="nofollow ugc">Reinventia</a>, Swedish – <a href="http://www.umu.se" rel="nofollow ugc">Hugo</a>, Turkish – Tony, Ukranian – <a href="http://www.everycloudtech.com/" rel="nofollow ugc">Ivanka</a>. Some of these translations are a bit out of date – updates welcome!

Conditional CAPTCHA

Asks commenters to complete a simple CAPTCHA if they don't have a previously approved comment, or if Akismet thinks their comment is spam.

WP-NOTCAPTCHA vs Conditional CAPTCHA