Fight With Comment Spam
Akismet checks your comments against the Akismet web service to see if they look like spam or not and lets you review the spam it catches under your blog’s “Comments” admin screen. Antispam plugin from the creators of WordPress which was add in default of wordpress packing and it require API key from WordPress.com and it’s free for personal use.
WP Captcha-Free blocks automated comment spam without resorting to CAPTCHAs. It does so by validating a hash based on time (and some other parameters) using AJAX when the form is posted. Comments posted via automated means will not have a hash or will have an expired hash and will be rejected. Unlike using a captcha, this does not place any burden on the commenter.
This plugin adds CAPTCHA anti-spam methods to WordPress forms for comments, registration, lost password, login, or all. In order to post comments or register, users will have to type in the code shown on the image. This prevents spam from automated bots. Adds security. Works great with Akismet. Also is fully WP, WPMU, and BuddyPress compatible.
Math Comment Spam Protection asks the visitor making the comment to answer a simple math question. This is intended to prove that the visitor is a human being and not a spam robot. its very handy usable plugin.
Defensio is an advanced spam filtering web service that learns and adapts to your behaviors and those of your users. In addition to simple spam filtering, we also provide world-class malicious content detection, profanity filtering, URL categorization, script detection and much more.