CleanTalk - Spam protection plugins for Joomla, WordPress, phpBB3, IP.Board, vBulletin and etc.
reCAPTCHA - reCAPTCHA is a free security service that protects your websites from spam and abuse.
Akismet - Akismet is a spam fighting service that protects millions of WordPress sites from comment and contact form spam.
OOPSpam Anti-Spam API - The better, new standard to stop spam content.
StopForumSpam - StopForumSpam - a database of known forum and blog spam, its sources and the email addresses reported
hCaptcha - Do you use a captcha to keep out bots? hCaptcha is a drop-in replacement for reCAPTCHA that earns website owners money and helps companies get their data labeled.