Google reCAPTCHA is becoming popular since the day of its announcement. The performance of this product is not been evaluated yet because of its age.
I have covered basic tutorial about how to code custom script with Google reCAPTCHA.
/*Add Google captcha field to Comment form*/
echo '<div class="g-recaptcha" data-sitekey= "=== Your site key === "></div>';
/*End of Google captcha*/