http://spamfizzle.com/CAPTCHA.aspx
This, of course, is not the only way to do it. The following method should be highly effective!
http://random.irb.hr/signup.php
Why do all this? Well, according to SJVN, the standard CAPTCHA is now practically useless.
http://www.pcworld.idg.com.au/index.php/id;489635775