CAPTCHA (challenge-response test for Drupal)

homepage
http://drupal.org/project/captcha

download
http://ftp.drupal.org/files/projects/captcha-6.x-2.1.tar.gz
extract
upload to /sites/all/modules

enable moulde
/admin/build/modules
# Go to Administer > User Management > CAPTCHA
# Check "Add Add CAPTCHA administration links to forms." option.