CleanTalk - Spam protection plugins for Joomla, WordPress, phpBB3, IP.Board, vBulletin and etc.
Antispam Bee - Say Goodbye to comment spam on your WordPress blog or website.
Akismet - Akismet is a spam fighting service that protects millions of WordPress sites from comment and contact form spam.
StopForumSpam - StopForumSpam - a database of known forum and blog spam, its sources and the email addresses reported
Mollom - Mollom is a freemium web service to get rid of spam, unwanted content and fake users on your...
Sblam! - Open-source implementation of server-side spam filtering of blog comments and forum posts.