reCaptcha has new plugin for MovableType. It is more easy when install it on MovableType 4.23. reCaptcha.7z The following installation howto includes MoveableType 4.0, 4.1. and 4.2. I applied it on my MovableType 4.23. It works perfect.
reCAPTCHA is better than MT native captcha image. reCAPTCHA is not only the a program that can tell whether its user is a human or a computer, but also let users help to recognize the scanned document. More details about the theory of reCAPTCHA. MovableType 4.1 has an native CAPTCHA