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.

Arnan de Gans

Tired of spam bots in your WordPress and ClassicPress website? Do you want to get rid of false registrations and other spammy nonsense? Don’t wan’t to use a clumsy and user-unfriendly Captcha? Don’t want to use a Captcha from Google or other big-tech company period?
Meet No-Bot Registration, easy to use, superior protection without making it hard for your visitors. Easily blacklist (partial) email addresses and domains so they can no longer register an account.
Create one or more questions and a set of possible answers for them and visitors have to answer your question when they register. 
If they answer wrong, they get denied their account.
Questions can be as simple as “1 + 1”, with possible answers being 1, one or uno. That way you can plan for eventualities and how people interpret your question.
<h4>Features</h4>
<ul>
<li>Protect registration forms</li>
<li>Protect the WooCommerce checkout form if you let people register from there</li>
<li>Protect your blog comment form</li>
<li>Prevents comment spam, trackback spam and other nuisances with ease</li>
<li>Set up multiple security questions to further confuse bots</li>
<li>Blacklist any email, domain or tld you don’t like</li>
<li>Configurable notification messages for users failing the security tests</li>
</ul>

No-Bot Registration

Prevent bots from creating accounts by blacklisting domains and usernames and present people with a human friendly security question.

Conditional CAPTCHA vs No-Bot Registration