Sabaoh

This plugin will add some reCAPTCHA widget to login form, comment form, and user registration form. With this plugin, sites owners can avoid spam comment, user registration, and biting password.
Mighty CAPTCHA uses a Google reCAPTCHA technology. To work, API key pair, issued Google, is necessary.
For more information about key pair, please refer https://www.google.com/recaptcha/intro/index.html .
You can choose which form will be with reCAPTCHA widget or not. For login form and user registration form, a normal size widget is too wide. So you can choose compact widget. (but I do not like it.)
Below is characteristic of new Google reCAPTCHA.
<ul>
<li>Easy for ordinary users. They only must check the “I’m not a robot”.</li>
<li>When Google reCAPTCHA recognized an access was smell fishy, image authentication screen would appear.</li>
<li>New image authentication screen is without deformed letters, with photo images instead of them.</li>
<li>Photo images authentication is for example “choose all photos of a cat”. Easy to human and hard to robot.</li>
<li>It’s easy to use with smart phone or tablet.</li>
</ul>
Why don’t you usher it into your site!

Mighty CAPTCHA

Mighty-CAPTCHA add an authentication with Google reCAPTCHA technology to login, comment, and register form, with API keys which delivered by Google.

Paolo

AyeCode

Stiofan

ReCaptcha addon for <a href="https://wordpress.org/plugins/userswp/" rel="ugc">UsersWP</a>.
The UsersWP ReCaptcha plugin allows you to implement a super security captcha into forms like 
registration, login forms. UsersWP Re-Captcha protects your site against spam, malicious registrations 
and other forms of attacks where computers try to disguise themselves as a human.
UsersWP Re-Captcha provides some of the smartest anti-spam security by protecting your site against spam and 
malicious registrations available today for WordPress.
<a href="https://www.google.com/recaptcha/" rel="nofollow ugc">Learn how reCAPTCHA works</a>
<h3>Requirements</h3>
<ul>
<li>You need the Google reCAPTCHA keys from <a href="https://www.google.com/recaptcha/admin#whyrecaptcha" rel="nofollow ugc">here</a>.</li>
</ul>

Mighty CAPTCHA vs UsersWP – ReCaptcha