CleanTalk - Spam protection for WordPress, Joomla, Drupal, and any other websites
StopForumSpam - StopForumSpam - a database of known forum and blog spam, its sources and the email addresses reported
Akismet - Akismet is a spam fighting service that protects millions of WordPress sites from comment and contact form spam.
Sblam! - Open-source implementation of server-side spam filtering of blog comments and forum posts.
reCAPTCHA - reCAPTCHA is a free security service that protects your websites from spam and abuse.
OOPSpam Anti-Spam API - The better, new standard to stop spam content.