f13dev

Add Google reCaptcha to the comments section on blog posts. Requires Google reCaptcha v2 Checkbox API key.
Simple configuration via the admin settings page: 
* reCaptcha public key 
* reCaptcha private key 
* enable reCaptcha for (Everyone | Visitors | Nobody [disabled])
Additional hooks for programmers: 
$v = apply_filters(‘f13_recaptcha_add’); 
Will place a reCaptcha checkbox in the desired place.
$validate = apply_filters(‘f13_recaptcha_validate’); 
if (!empty($validate)) { 
 $v = $validate 
 // reCaptcha failed 
} else { 
 // reCaptcah passed 
}

F13 reCaptcha

Add Google reCaptcha to the comments section on blog posts. Additional hooks for adding reCaptcha to custom forms.

Beherit

Plugin protects WordPress website forms from spam and brute-force attacks with Google reCAPTCHA. There is no additional user-unfriendly options, just choose captcha version, enter keys and optionally set captcha theme, size or badge position – nothing more!
<h4>What forms are protected with reCAPTCHA?</h4>
<ul>
<li>Login form</li>
<li>Reset password form</li>
<li>New password form</li>
<li>Registration form</li>
<li>Comment form</li>
<li><a href="https://wordpress.org/plugins/lite-contact-form/" rel="ugc">Lite Contact Form</a></li>
<li>Contact Form 7</li>
</ul>
The reCAPTCHA verification will be required only for not logged in users.

GreCAPTCHA

Protects WordPress website forms from spam and brute-force attacks with Google reCAPTCHA.

F13 reCaptcha vs GreCAPTCHA