yaCAPTCHA is a CAPTCHA plugin for WordPress that helps you block comment spam 
from automated bots. In order to post comments, users will have to write down 
the characters that are part of an image. Since it is relatively hard for 
automated programs to figure out those characters, this will help prevent 
comment spam from those programs.
<h4>Requirements</h4>
<ul>
<li>WordPress 1.5 or higher.</li>
<li>PHP 4.1.0 or higher with GD2 library support.</li>
<li>Theme must support the ‘comment_form’ action.</li>
<li>PHP sessions enabled and properly configured.</li>
</ul>
In case you are wondering, most hosting providers should have those basic 
requirements in place.
<h4>Strengths vs other solutions</h4>
<ul>
<li>Easy to install</li>
<li>Does not require Javascript to work</li>
<li>Broad WordPress version support</li>
</ul>
<h3>Thanks</h3>
Thanks to Kruglov Sergei for creating KCAPTCHA, his pretty good CAPTCHA. You 
can visit KCAPTCHA website <a href="http://www.captcha.ru/en/kcaptcha/" title="KCAPTCHA
website">here</a>.

yaCAPTCHA

yaCAPTCHA is a CAPTCHA plugin for WordPress that helps you block comment spam from automated bots.

MihChe

WPBruiser (formerly GoodBye Captcha) is an anti-spam and security plugin based on algorithms that identify spam bots without any annoying and hard to read captcha images.
WPBruiser completely eliminates spam-bot signups, spam comments, even brute force attacks, the second you install it on your WordPress website. It is completely invisible to the end-user – no need to ever fill out a Captcha or other “human-detection” field ever again – and it just works!
Unlike other anti-spam plugins, which detect spam comments and signups after the fact and move them to your spam folder, which you then have to delete – using up not only your website’s resources, but your time as well, WPBruiser prevents the bots from leaving spam in the first place. The result is that your site is not only spam free, it’s faster and more secure.
In addition, WPBruiser is completely self-contained and does not need to connect to any outside service. Your logins remain yours, 100%.
WPBruiser fights Brute Force attacks and eliminates spam-bots on comments, signup pages as well as login and password reset pages. At the click of a button, you can decide which forms to protect.
<h4>Summary of WPBruiser features</h4>
<ul>
<li>Standard WordPress Login form integration</li>
<li>Standard WordPress Register form integration</li>
<li>Standard WordPress Forgot Password form integration</li>
<li>Standard WordPress Comments form integration</li>
<li>Ability to set the maximum number of characters for each comment field</li>
<li>Logging with the ability to enable/disable it</li>
<li>Automatically Block IP Addresses</li>
<li>Automatically purge logs older than a certain number of days</li>
<li>Manually white-list trusted IP Address (IPV4 and IPV6)</li>
<li>Manually block/unblock IP Addresses (IPV4 and IPV6)</li>
<li>Properly detects client IP Address when using CloudFlare, Incapsula, Cloudfront, RackSpace, Sucuri CloudProxy, AWS ELB</li>
<li>Provides statistics, reports, maps and charts with all blocked spam attempts</li>
<li>No requests to external APIs</li>
<li>Can be switched to “Test Mode” – for testing</li>
<li>Compatible with WordPress Multisite – network admin interface ready</li>
<li>Compatible with cache plugins (WP Super Cache, W3 Total Cache, ZenCache, WP Fastest Cache and others)</li>
<li>Invisible for end users (works in the background)</li>
<li>Does not affect page loading times</li>
</ul>
<h4>Brute Force Protection</h4>
<ul>
<li>Automatically detects Brute Force attacks</li>
<li>Ability to automatically block IP Addresses</li>
<li>Prevents User Enumeration</li>
<li>Ability to block most dangerous IP addresses involved in brute force attacks</li>
<li>Ability to block most dangerous Anonymous Proxy IP addresses including TOR Networks, TOR Nodes and TOR Exit Points</li>
<li>Ability to Completely Disable XML-RPC service – it seamlessly works with Jetpack plugin activated</li>
<li>Ability to Disable XML-RPC Pingbacks</li>
<li>Email notifications when a Brute Force Attack is detected</li>
</ul>
<blockquote>
<h4>WPBruiser Available Extensions</h4>
WPBruiser is integrated with the most popular plugins
<dl>
<dt>Contact Forms Extensions</dt>
<dd>
<ul>
<li><a href="http://www.wpbruiser.com/downloads/contact-form-7/" rel="nofollow ugc">WPBruiser – Contact Form 7</a></li>
<li><a href="http://www.wpbruiser.com/downloads/gravity-forms/" rel="nofollow ugc">WPBruiser – Gravity Forms</a></li>
<li><a href="http://www.wpbruiser.com/downloads/ninja-forms/" rel="nofollow ugc">WPBruiser – Ninja Forms</a></li>
<li><a href="http://www.wpbruiser.com/downloads/formidable-forms/" rel="nofollow ugc">WPBruiser – Formidable Forms</a></li>
<li><a href="http://www.wpbruiser.com/downloads/fast-secure-contact-form/" rel="nofollow ugc">WPBruiser – Fast Secure Contact Form</a></li>
<li><a href="" rel="nofollow ugc">WPBruiser – Jetpack Contact Form (FREE – merged into the core)</a></li>
</ul>
</dd>
</dl>
<dl>
<dt>Membership Extensions</dt>
<dd>
<ul>
<li><a href="http://www.wpbruiser.com/downloads/buddypress/" rel="nofollow ugc">WPBruiser – BuddyPress</a></li>
<li><a href="http://www.wpbruiser.com/downloads/memberpress/" rel="nofollow ugc">WPBruiser – MemberPress</a></li>
<li><a href="http://www.wpbruiser.com/downloads/userpro/" rel="nofollow ugc">WPBruiser – UserPro</a></li>
<li><a href="http://www.wpbruiser.com/downloads/upme/" rel="nofollow ugc">WPBruiser – User Profiles Made Easy</a></li>
<li><a href="" rel="nofollow ugc">WPBruiser – Ultimate Member (FREE – merged into the core)</a></li>
</ul>
</dd>
</dl>
<dl>
<dt>eCommerce Extensions</dt>
<dd>
<ul>
<li><a href="http://www.wpbruiser.com/downloads/woocommerce/" rel="nofollow ugc">WPBruiser – WooCommerce</a></li>
<li><a href="http://www.wpbruiser.com/downloads/easy-digital-downloads/" rel="nofollow ugc">WPBruiser – Easy Digital Downloads</a></li>
<li><a href="http://www.wpbruiser.com/downloads/affiliatewp/" rel="nofollow ugc">WPBruiser – AffiliateWP</a></li>
</ul>
</dd>
</dl>
<dl>
<dt>Email Subscriptions Extensions</dt>
<dd>
<ul>
<li><a href="http://www.wpbruiser.com/downloads/mailpoet/" rel="nofollow ugc">WPBruiser – MailPoet</a></li>
<li><a href="http://www.wpbruiser.com/downloads/easy-forms-for-mailchimp/" rel="nofollow ugc">WPBruiser – Easy Forms for MailChimp</a></li>
</ul>
</dd>
</dl>
<a href="http://www.wpbruiser.com/extensions/" title="WPBruiser Extensions" rel="nofollow ugc">View all WPBruiser Extensions</a>
</blockquote>
WPBruiser is also integrated with the following plugins:
<ul>
<li>
Postmatic (https://wordpress.org/plugins/postmatic) 
WPBruiser offers protection for the entire email commenting system
</li>
<li>
Epoch (https://wordpress.org/plugins/epoch) 
WPBruiser offers protection for the entire chat and commenting system
</li>
<li>
wpDiscuz (https://wordpress.org/plugins/wpdiscuz/) 
WPBruiser offers protection for the entire commenting system
</li>
<li>
MailChimp for WordPress (https://wordpress.org/plugins/mailchimp-for-wp) 
WPBruiser offers protection for all forms the user will create with MailChimp
</li>
<li>
Ultimate Member (https://wordpress.org/plugins/ultimate-member) 
WPBruiser offers protection for Login, Registration and Reset Password forms
</li>
<li>
Jetpack by WordPress (https://wordpress.org/plugins/jetpack) 
WPBruiser offers protection for JetPack Contact Form
</li>
<li>
ZM Ajax Login & Register (https://wordpress.org/plugins/zm-ajax-login-register) 
WPBruiser offers protection for Login and Registration forms
</li>
<li>
Login With Ajax (https://wordpress.org/plugins/login-with-ajax) 
WPBruiser offers protection for Login, Registration and Lost Password forms
</li>
<li>
WP User Control (https://wordpress.org/plugins/wp-user-control) 
WPBruiser offers protection for Login, Registration and Lost Password forms
</li>
<li>
PlanSo Forms (https://wordpress.org/plugins/planso-forms/) 
WPBruiser offers protection for all forms
</li>
<li>
Theme My Login (https://wordpress.org/plugins/theme-my-login) 
WPBruiser offers protection for Login, Registration and Lost Password forms
</li>
<li>
Seamless Donations (https://wordpress.org/plugins/seamless-donations) 
WPBruiser offers protection for the donation form
</li>
</ul>
<h4>Technical support</h4>
If you notice any problems by using this plugin, please notify us and we will investigate and fix the issues. Ideally your request should contain: URL of the website (if your site is public), Php version, WordPress version and all the steps in order to replicate the issue (if you are able to reproduce it somehow)
<h4>Donate</h4>
If you find this plugin useful, please consider making a small <a href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=XVC3TSGEJQP2U" rel="nofollow ugc">donation</a>. Thank you

WPBruiser {no- Captcha anti-Spam}

An extremely powerful antispam plugin that blocks spam-bots without annoying captcha images.

yaCAPTCHA vs WPBruiser {no- Captcha anti-Spam}