AntiSpam. AntiSpam module is the successor of the Akismet module, and it provides spam protection to your drupal site using external antispam service like Akismet.
AntiSpam module is fully compatible with Drupal 6.x (Akismet module for Drupal 6.x release had many compatibility issues and was not usable as it was), and it expanded the support of the external antispam service with TypePad AntiSpam and Defensio service as well as Akismet service. Now you can choose one of the antispam service you wish to use. Comment spam protection and CAPTCHA service. Mollom. Mollom is an intelligent content moderation web service.
By monitoring content activity on all sites in the Mollom network, Mollom is in a unique position to determine if a post is potentially spam; not only based on the posted content, but also on the past activity and reputation of the poster. In short, Mollom handles incoming posts intelligently, in much the same way a human moderator decides what posts are acceptable.