It is a given side effect of blogs that as soon as you have one, the spammers of the world find you’re out there and start posting inane and useless stuff as comments on your site. WordPress has a very good span filter in the form of Askimet, but it helps to keep the spammers out before they even get to post, but using anti-robot technology.

ReCaptcha

ReCaptcha

One such option is provided through the use of key words displayed as distorted images whenever a comment is requested. Known as CAPTCHA (for Completely Automated Turing Test To Tell Computers and Humans Apart) this technique was invented out of Carnegie Mellon University in 2000, and does a very creditable job. However, to gain extra value, this has been refined into reCAPTCHA, which utilizes CAPTCHA to assist in the process of digitizing the text of old books, thus generally adding to the amount of good information in electronic form.

To get more info, go to the reCAPTCHA site.