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.

Matt Keys

Anti-Spam solutions that require your users to fill out captcha’s are frustrating for actual humans, and not that great at stopping bots.
This solution works automatically without any user configuration, and puts no additional burden on your users. After activation this plugin will go to work stopping: comment spam, spam bot registration, and bbPress spam.
Key features:
<ul>
<li>Fully compatible with wordpress caching plugins or even more advanced server level caching solutions (like WP Engine uses).</li>
<li>Utilizes randomly generated and rotating token field name and token value, which stops more clever bots that might try caching the ‘solved’ field.</li>
<li>Anti-spam field is randomly placed within the form on page load to make it difficult for spammers to target the field.</li>
<li>Developer friendly with filters and actions</li>
<li>Lightweight, clean, and efficient solution to comment, registration, and bbPress spam</li>
<li>Uses native JavaScript to avoid jQuery dependancies</li>
</ul>
Requires JavaScript be enabled in client browser (Users will see a warning if JavaScript is disabled)

Advanced Invisible Anti-Spam

Block bots without annoying captchas. Cache friendly solution with rotating keys! Blocks comment, registration, and bbpress spam. Activate and done!

GreCAPTCHA vs Advanced Invisible Anti-Spam