A CAPTCHA is a challenge-response test most often placed within web forms to determine whether the user is human. The purpose of CAPTCHA is to block form submissions by spambots, which are automated scripts that post spam content everywhere they can. The CAPTCHA module provides this feature to virtually any user facing web form on a Drupal site.
We do this our spare time, which is unfortunately almost nonexistent at the moment due to real life obligations. To give the CAPTCHA module the required level of maintenance, an extra co-maintainer would be welcome. If you're interested in helping with this very popular module, please contact me or open an issue in the CAPTCHA module issue tracker.
You could also consider spam filtering or hybrid solutions like:
The AntiSpam module, is the successor of the Akismet module, and it provides spam protection to your Drupal site using external antispam service like Akismet, TypePad and Defensio.
The Mollom module, similar to AntiSpam module, it only presents a CAPTCHA if the user input (e.g. a comment) looks like spam. Also provides stand-alone CAPTCHA service with intelligent, reputation-based CAPTCHAs. It is restricted to the Mollom service