If your Drupal 'Captcha' isn't working, in that it's displaying a lovely background pattern, but no actual captcha text for the user to solve, you should check your captcha settings at

http://<yoursite>/admin/user/captcha/image_captcha

After a recent site upgrade, the font settings for the Captcha were cleared and no font = no captcha. Select a font and "Hey, Presto!" it all works again as expected.