WordPress – Adding ReCAPTCHA
2016 Note: We no longer use ReCAPTCHA on our WordPress sites. Instead we use a plugin called WP-SpamShield. We have found it 100% effective in eliminating spam. Stop reading now and install WP-SpamShield!
Adding reCAPTCHA to your WordPress installation is one way to reduce spam. It forces users to enter a word or string of numbers in order to submit a comment or contact form from your website. The idea is that robots will not be able to recognize the information that a human user can easily recognize and type.
To learn all about it, start here: www.google.com/recaptcha/intro/