First time I heard about reCAPTHCA (Blognone entry), I thought it's very brilliant idea. Protect your site from spam comments and benefit whole humanity in the same click, never know cool thing like this. Only one barrier that keeps me out of using it is no convenient module for Drupal.
Now, that barrier is gone. Someone has been contributed reCAPTCHA module for Drupal already. All you need to do is: