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.

michael_greenberg

PHPEnkoder is a port of the excellent <a href="http://hivelogic.com/enkoder" rel="nofollow ugc">Hivelogic 
Enkoder</a> to PHP and, more specifically, 
to WordPress. It is used to display text in a way that users can see 
and bots can’t.
The encoding system is directly and unabashedly stolen from the 
BSD-licensed source of Hivelogic Enkoder, which works by randomly 
encoding a piece of text and sending to the browser self-evaluating 
Javascript that will generate the original text. This works in two 
ways: first, a bot must first have a fairly complete Javascript 
implementation (in particular, it must have <code>eval</code>); second, the 
decoding process can be made arbitrarily computationally 
intensive. This is similar to the idea of charging computational 
payments to send e-mail, only this is actually implemented.
By default, PHPEnkoder scrambles e-mails in plaintext and in <code>mailto:</code> 
links. It additionally provides a shortcode for manual scrambling, 
used like so: <code>[enkode text="shown to non-JS browsers"]this will be 
scrambled[/enkode]</code>.

PHPEnkoder

Encodes mailto: links and e-mail addresses with JavaScript to stifle webcrawlers.  Automatically turns plaintext e-mails into (enkoded) links.

GreCAPTCHA vs PHPEnkoder