What you are referring to is called CAPTCHA. There are many different ones you can find (a google search should turn up quite a lot of results), but you may want to try
http://captcha.net/ http://recaptcha.net/
Both of these are similar, the first link is to a system that generates an image, the second uses scanned texts from books which failed to be correctly read by automated programs. By using recaptcha you are helping to digitize these words. Also both sites give good documentation on how to add it to your site
Hope that helps