CAPTCHA provides a simple yet effective way to tell humans and computers apart. The need of CAPTCAH arises because web pages which accept user input such as posting comment are vulnerable to automated spam. It is not a difficult task to write a script that could generate large amount of repetitive information and post them to a targeted page in short period of time. Unless the page is protected, it will soon polluted by those spam.
For instance, the site you are visiting, Code Central, receives over 10 junk comments everyday. Fortunately, WordPress uses an algorithm that can filter out most of these junk comments and flags them for review. The unfortunate part is that administrating them is laborious and frustrating - I don't want to delete them in a batch because I don't want to accidentally delete a comment that is submitted by a real user. So I decided to install a CAPTCHA to see if that is going to help.
After some research, I found this - Protect Web Form. The site offers CAPTCHA service for free. It only took me a few minutes to set it up as a WordPress plugin. So far it stops those automated comments nicely. Further customizations such as word length and background color can be made through the web interface. Overall, the quality of this script is good. If you need a CAPTCHA script for your blog, consider this one.