Mollom is Drupal's Automated Spam Detection Tool, and it is brilliant !

Mollom is a module that requires an "API" key, a specific Random string of characters to communicate between a website and central reporting server.

Mollom is a new service started by Benjamin Schrauwen and Dries Buytaert, creator and project lead of Drupal. This automatically scans the content of comments and preventing obvious spam from even being posted to the website.

Mollom support blocking not just comment spam but also spam from the contact form
blog entries, forum topics, user registration , password request form. This is unique to Mollom that is why I also uses this with my clients. For me this is a one-stop solution, rather than having one tool for blogs and another tool for handling comments.

Now sit back and relax, your site should now reduce the amount of automatic spam to zero ! Job done !