Akismet
A spam filtering program that is free for personal use and you have to pay for corporate usage.
Sblam!
Open-source implementation of server-side spam filtering of blog comments and forum posts. It's available as a free web servic ...
CleanTalk
Spam protection plugins for Joomla, WordPress, phpBB3, IP.Board, vBulletin and etc. Protection without CAPTCHA.
Mollom
Mollom is a freemium web service to get rid of spam, unwanted content and fake users on your Drupal, Wordpress and Joomla site ...