http:BL

From the project page, "Implementation of http:BL for Drupal. http:BL can prevent email address harvesters and comment spammers from visiting your site by using a centralized DNS blacklist. It requires a free Project Honey Pot membership. This module provides efficient blacklist lookups and blocks malicious visitors effectively.

Mollom

Make spam go away! This is a great service that is free for a low traffic site. A great anti-spam solution for Drupal! Also check out Honeypot and the Bad Behavior module.