In BOTCHA, we don't abuse our human users - BOTCHA protection is completelytransparent to them and non-intrusive.
The CAPTCHA Pack module contains several CAPTCHA types for use with the CAPTCHA module. The CAPTCHA Pack module is meant to provide lightweight, yet effective alternatives for the traditional image CAPTCHA, which is undesirable in certain situation (e.g. bandwidth restrictions, cpu restrictions, accessibility constraints, etc).
Mollom provides a one stop solution for all spam problems and can protect the following Drupal forms
Uses the reCAPTCHA web service to improve the CAPTCHA system and protect email addresses.