Generally reCAPTCHA Public captcha, which in its time was purchased Google (if I understood correctly), and has been developing under the wing of the online giant. This is one of the most popular scripts used in many CMS and services, it is also almost the most protected from bots.
WP-reCAPTCHA . Setting his classic, but after activating the system you will need to get a special key . On the same page, by the way, you will find all the necessary information for developers, which deals with reCAPTCHA.
In the module settings WP-reCAPTCHA you can choose: activation kapchti for comment and / or a registration form, appearance (theme) capchti, language, HTML standard display code, the error message text. The only thing that confused - support to Wordpress version 2.9.2 and the latest update in early 2012, but nearly 400,000 downloads.
SI CAPTCHA Anti-Spam
Plugin SI CAPTCHA Anti-Spam is most often found in various compilations - as I understand it is the most popular module for captchas. It allows you to add a security code to all (or selected) forms in WordPress - registration, forgotten password, login, comments. No problem is compatible with Akismet, and also works in WPMU and BuddyPress .
As for the features of the plug-SI CAPTCHA Anti-Spam, we find there: different settings, valid HTML code to display / show the security code for different forms or users logged on or not (useful when commenting.) Support localization. The module works with WordPress 2.9 to the latest version, the current update of 06.01.2013 and 1.5 million downloads.
A distinctive feature of the Captcha module is the use of different mathematical equations. This will protect your site is not only on boats, but also protect against inappropriate and unreasonable people (who sometimes come across). Traditionally, you can configure it to any form of the system - from check-in to comment. Available in almost all language versions. Requirements for WordPress - from version 2.9, the latest release - 08/02/2013 and over 400 thousand downloads.
Saved the last more or less new to me captcha that have not yet met - Secure CAPTCHA . In the image of the security code using the writing. Because of this, the program decryptor difficult to separate different letters to crack CAPTCHA, which will only be understood by humans. In addition, for all words can be used for some additional conversion in order to confuse bots.
According to the developers at the expense of what the letters stand out well from the background, no need to look into particular image. Although, frankly, I sometimes still hard to understand what is written there. To use this capchtu, after installation and activation you will need to register on the site developers and get special keys. Then in the settings choose what kind of shape you want to protect against spam. Module is downloaded, not many people - only 5200, but in general the idea is interesting enough. Need to work WordPress version 3.1 and above.
Of course, this is not all plugins captcha in WordPress, of which there are a lot more, and there are far more sophisticated and unusual - the folding part of the picture (as in the puzzle), the choice of a special symbol shown in captchas, etc. I mentioned in a post, only those that are most frequently mentioned in the compilations of various modules + immediately found in the search. The first 4 - definitely a find, I would happily use each of them on their own and developed sites.