Wordfence - Comprehensive security plugin for WordPress.
CleanTalk - Spam protection for WordPress, Joomla, Drupal, and any other websites
iThemes Security - Security plugin that provides over 30+ ways to secure and protect your WordPress site.
Akismet - Akismet is a spam fighting service that protects millions of WordPress sites from comment and contact form spam.
NinjaFirewall (WP Edition) - A true Web Application Firewall to secure and protect WordPress.
reCAPTCHA - reCAPTCHA is a free security service that protects your websites from spam and abuse.