Akismet by Automattic
Akismet is an networked anti-spam web service. It is currently in used to pre-screen blog comments and trackbacks, bulletin board submissions, and requests for new user accounts. Akismet learns about spam from the actions of its distributed user base, leveraging the wisdom of the crowd to flag spam sources and their messages. Akismet is free for non-commercial use, $5 a month for pro-bloggers, and $50 for enterprise customers.