Jehy

<a href="https://wordpress.org/plugins/securimage-wp-fixed/" rel="ugc">Securimage-WP-Fixed</a> utilizes the powerful CAPTCHA protection of <a href="http://phpcaptcha.org/" title="Securimage PHP CAPTCHA" rel="nofollow ugc">Securimage Captcha</a> which adds protection to your WordPress comment forms. Unfortunately, it didn’t have option to be installed on user register page. Now you can just install Securimage-WP-REG, enable it – and you will have CAPTCHA on register page!
So, Securimage-WP-REG adds captcha on register pages.
Requirements:
<ul>
<li><a href="https://wordpress.org/plugins/securimage-wp-fixed/" rel="ugc">Securimage-WP-Fixed</a> plugin installed and working. Old version of this plugin you can find <a href="https://www.phpcaptcha.org/download/wordpress-plugin/" rel="nofollow ugc">here</a>.</li>
</ul>
<h4>Donate or help?</h4>
If you want to ensure the future development and support of this plugin, you can make donation <a href="http://jehy.ru/articles/donate/" rel="nofollow ugc">on this page</a> or just write about this plugin in your blog.

Securimage-WP-REG

Securimage-WP-REG adds captcha ptotection from Securimage-WP plugin on register pages.

Silkalns

WP Chill

Cristian Raiber

A very useful plugin for everyone using WordPress. Adds reCaptcha security to the WordPress login form, register form and comment form. This plugin could help your blog get rid of a lot of spam comments or brute-force attacks.
Nothing gets passed it if the reCaptcha doesn’t validate.
A few notes about the plugin:
<ul>
<li>Supports audio or image captcha types</li>
<li>Can generate the reCaptcha image / audio type in a number of predefined languages</li>
<li>Adds reCaptcha protection to the WordPress login form</li>
<li>Adds reCaptcha protection to the WordPress register form</li>
<li>Adds reCaptcha protection to the WordPress comment form</li>
<li>Adds reCaptcha protection to the WordPress recover password form</li>
</ul>

Uber reCaptcha

This plugin adds the reCaptcha form to the WordPress login form, recover password form, register form and comment form.

Securimage-WP-REG vs Uber reCaptcha