Captcha Code - This plugin can be used to add captcha code protection on various types of logins and forms active on a typical WordPress site. The configuration of this powerful plugin is quite easy. This plugin is completely translation-ready.
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 web forms. Key CAPTCHA also uses unique captcha monetization system (optional) that can be used to make some money. It can be used in two modes viz., Flash and HTML5 and is fully compatible with BuddyPress.
Cartpauj Register Captcha - This handy plugin adds a captcha challenge to the use registration form of the site. It also adds honeypots in hidden format to deter various kinds of spam bots. and last but not the least, it doesn't require any kind of complex setup to make it work. Just install it and activate it.
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 are 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 It was originally made to work with much popular Contact Form 7 plugin.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.
Google Captcha - This anti-spam plugin adds effective captcha solution offered by Google to various types of forms on a WordPress blog. It also supports Contact Forms 7. It is multilingual with support for over a dozen popular languages and also comes with RTL support.
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 to the visitor. You'll need a publisher key to use this plugin. It can be used on custom web 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.