Stopping Drupal 7 spammers registering at mysite.com
There is Honeypot as well 


  Honeypot uses both the honeypot and timestamp methods of deterring
  spam bots from completing forms on your Drupal site


Usually, forms take at least a few seconds to fill out when a human is entering data into them—especially surveys, user registration forms, etc. Spam bots try to fill out as many forms as they can in as little time as possible, so they will often fill out a form within a couple seconds at most. The Honeypot module requires at least 5 seconds to pass (by default - you can adjust this too!) before a form can be submitted.

Alternative modules

There are many spam-protection modules on drupal.org, some of which have more or less robust methods of dealing with form spam. Some of the most used and recommended include:

Mollom - Offers spam protection through their premium service.
CAPTCHA - A simple image-based CAPTCHA builder.
reCAPTCHA - Implements reCAPTCHA for image-based CAPTCHAs.
Hidden CAPTCHA - Similar to this module in it's effectiveness.