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.

konnektiv

This Plugin allows administrators to restrict user login based on user roles. This is useful e.g. when you want anyone to register on your site but you do only want certain people (e.g. editors) to enter the logged in area.
Roles that are allowed to log in are set in a new menu entry in the Users menu.
This plugin was originally developed for the <a href="https://quality4digitallearning.org/" rel="nofollow ugc">globe – Community of Digital Learning</a> on behalf of <a href="https://www.giz.de/" rel="nofollow ugc">GIZ</a>.
<h4>Contact</h4>
<ul>
<li><a href="http://konnektiv.de/" rel="nofollow ugc">Konnektiv</a></li>
<li><a href="https://github.com/konnektiv/restrict-role-login" rel="nofollow ugc">Restrict Role Login on GitHub</a> – Report issues, contribute code</li>
</ul>

Restrict Role Login

Allows administrators to restrict user login based on user roles.

Mighty CAPTCHA vs Restrict Role Login