Chapter 10 describes the Honeypot and CAPTCHA modules, two very useful tools in reducing spam accounts and content on your site.

SpamSpan Filter is another useful module if you use the Email Field module to display email addresses. It uses Javascript to make the email addresses harder for spammers to harvest.

Mollom is another product created by the founder of Drupal that uses machine learning to predict whether content is spam or not. It analyzes text submitted to your site (e.g. as a comment or a node), evaluates it, and if Mollom is unsure whether it’s spam or not, it will display a CAPTCHA. It’s actively maintained by Acquia, a major Drupal development and hosting company.