In my blog, I use a captcha to check comments are not spam. Captcha is a type of challenge-response test used in computing to determine whether or not the user is human. This is a good mean to filter comments and easy to use, generally… However, some people are very worried about too much traffic, sometimes too much worried. Here is a list of craziest captchas on the web.
I particulary like the russian captcha. And you ?