reCAPTCHA is an anti-spam method originating from Carnegie Mellon University which uses CAPTCHAs in a genius way. Instead of randomly generating useless characters which users grow tired of continuosly typing in, risking the possibility that spammers will eventually write sophisticated spam bots which use OCR libraries to read the characters, reCAPTCHA uses a different approach. While the world is in the process of digitizing books, sometimes certain words cannot be read. reCAPTCHA uses a combination of these words, further distorts them, and then constructs a CAPTCHA image. After a ceratin percentage of users solve the 'uknown' word the same way it is assumed that it is the correct spelling of the word. This helps digitize books, giving users a reason to solve reCAPTCHA forms. Because the industry level scanners and OCR software which are used to digitize the books can't read the words with which the CAPTCHAs are constructed, it is safe to assume that in-house spam-bot OCR techniques will not be able to bypass the CAPTCHA either. reCAPTCHA has earned a very prestigious reputation among the various CAPTCHA systems available and is used by such sites as Facebook, Twitter, StumbleUpon, and a few U.S. Government Websites such as the TV Converter Box Coupon Program Website. You can get the plugin here. TO INSTALL AS FORCED ACTIVATED/SITEWIDE: First, upload the wp-recaptcha folder to your mu-plugins folder then move the wp-recaptcha.php file out of the wp-recaptcha folder so that it's directly at mu-plugins/wp-recaptcha.php. TO INSTALL AS OPTIONALLY ACTIVATED BY USERS: Install it as if you were going to install it for regular wordpress (wp-content/plugins).