wppal

Easy Captcha is an ideal solution for those who want to try different types of captcha to achieve an optimal site usability and captcha protection ratio. It allows a WordPress site owner to choose which captcha should be used on a particular page.
Currently the plugin supports 3 types of captcha:
<ul>
<li>Hidden captcha, </li>
<li>Simple image captcha,</li>
<li>Google reCaptcha</li>
</ul>
Hidden Captcha is invisible to site visitors and does not assume any specific user actions. It just checks that a visitor behaves like a human. Although it might seem that Hidden Captcha does not fully correspond to the definition of CAPTCHA (found at the end of the FAQ), it still distinguishes a human from a bot based on natural human behavior. Hidden Captcha also allows checking whether a visitor’s browser has JavaScript and Cookies enabled.
Simple Captcha works in a traditional manner: it shows an image and requires a user to enter a captcha code. Simple Captcha stores captcha values on the server side using a custom session approach.
reCaptcha is a reliable and powerful captcha service provided by Google for free. Please visit <a href="http://google.com/recaptcha" rel="nofollow ugc">reCaptcha site</a> for more details
Any type of captcha from above may be added to the following WordPress site places:
<ul>
<li>Comment forms,</li>
<li>Login page,</li>
<li>Registration page,</li>
<li>Password reset page. </li>
</ul>
With Easy Captcha you can easily install different types of captcha on each individual page.
Please read the FAQ and Installation sections for more details.

Easy Captcha

Several types of captcha to protect your site in one easy to use plugin.

elrata_

NOTE: I’m looking for a new maintainer. Please reach out to 
rodrigo@sdfg.com.ar if you are interested.
NOTE: I don’t have wordpress 5 installations to try this and plan to stop 
updating it. If this is a big issue for you, please let me know so I can 
consider to continue doing it. Also, let me know if you are willing to help with testing :).
This is a small modification of the wp-recaptcha plugin to make it work with 
BuddyPress. Currently uses recaptcha v2.
You only need this plugin if you use Buddypress with wordpress and want a 
recaptcha at the registration page.
It is always rebased on top of wp-recaptcha. See 
https://github.com/rata/wp-recaptcha-bp for more info
What is reCAPTCHA?
reCAPTCHA is a free CAPTCHA service that protects your site against spam, malicious registrations and other forms of attacks where computers try to disguise themselves as a human; a CAPTCHA is a Completely Automated Public Turing test to tell Computers and Human Apart. reCAPTCHA comes in the form of a widget that you can easily add to your blog, forum, registration form, etc.
In addition to protecting your site, reCAPTCHA also helps us digitize old books and newspapers, transcribe street numbers and solve hard AI problems. <a href="http://www.google.com/recaptcha" rel="nofollow ugc">Learn how reCAPTCHA works</a> and join our <a href="https://groups.google.com/forum/#!forum/recaptcha" rel="nofollow ugc">forum</a>.
<h3>Requirements</h3>
<ul>
<li>You need the reCAPTCHA keys <a href="https://www.google.com/recaptcha/admin#whyrecaptcha" rel="nofollow ugc">here</a>.</li>
<li>Your theme must have a <code>do_action('comment_form', $post->ID);</code> call right before the end of your form (Right before the closing form tag). Most themes do.</li>
</ul>

WP-reCAPTCHA-bp

Integrates reCAPTCHA anti-spam methods with WordPress sites using BuddyPress plugin, including comment and registration spam protection.

Easy Captcha vs WP-reCAPTCHA-bp