WordPress has a fairly strong mechanism to deal with spammers and trolls. Yet we need some specialized plugins to deal with this problem. The most popular anti-Spam plugin Akismet is used on almost every WordPress blog. However, some trolls easily bypass this security and create unpleasant experience for the blog owner as well as for the community as a whole. Although there's no silver bullet to stop them, but captcha systems does act as a deterrent to a certain extent. Here's the collection of best captcha plugins for WordPress powered web sites and blogs.
WP-reCAPTCHA - It is perhaps the most popular captcha plugin with a large user base. It uses an image of distorted words that cannot be read easily from OCR software. This significantly reduces flaming or junk entries from spammers and trolls.
Contact Form With Captcha - It's one of the best plugins for implementing contact forms integrated with captcha system. It has a simple configuration system with editable email identifiers to create filters in your email system to better manage spam (if any) and normal entries.
SI CAPTCHA Anti-Spam - This is yet another popular captcha plugin used with forms. It easily integrates with comment, login and custom contact forms. SI CAPTCHA also supports WordPress multi-site installation and BuddyPress. You can also configure it to hide captcha system from logged in users.
Key CAPTCHA - This plugin is also quite popular among WordPress community. It easily integrates with comments, login and various custom forms. Key CAPTCHA also uses unique captcha monetization system (optional) that can be used to make some money. It can be used in two mode viz., Flash and HTML5 and is fully compatible with BuddyPress.
Sweet Captcha - This unique plugin employs a different approach in presenting captcha challenge. The visitor has to answer a question by dragging the correct answer from a set of answers. This awesome plugin works equally well on smartphones and tablet PCs. Sweet Captcha easily integrates with different types of custom forms.
Captcha - This is a simple yet very effective and lightweight captcha plugin for WordPress bloggers. Every visitor is presented with a mathematical challenge that ensures no bot or automated system penetrates the security. It may annoy some visitors but works effectively is deterring trolls and spammers.
Really Simple CAPTCHA - This plugin is used by large number of bloggers to combat trolls. It was originally made to work with much popular Contact Form 7 plugin. Users can easily integrate it with other plugins through simple PHP directives. This essentially means that you can easily extend the functionality of other plugins through this plugin.
NuCaptcha - And last but not the least is NuCaptcha anti-spam plugin that uses video captchas to combat trolls and spammers. It is fully compatible with BuddyPress and Contact Form 7 and can be easily deployed on various custom forms like native comment system, login and contact forms.
Are You a Human - It's a unique plugin that effectively deters spammers, bots and trolls. Unlike normal captcha systems that use distorted words, this plugin use games to present a challenge before the visitor. You'll need a publisher key to use this plugin. It can be used on custom forms as well as with the native comment system.
VouchSafe CAPTCHA Replacement - Nowadays new methodologies are being implemented to fight spam bots. And that's where plugins like these come handy. The innovative challenge system in VouchSafe plugin makes it almost impossible for machines to bypass the check. You'll need a publisher key to use this plugin on your blog.