When a new comment, trackback, or pingback comes to your blog it is submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down. There's no maintenance or upgrading needed and it comes with WordPress.
Challenge restricts comment spam by throwing the commenter a challenge. The default challenge is an addition and multiplication math question. The challenge and the answer can also be user-defined. They can be of anything including PHP-evaluatable expressions.
Use this plugin to bounce back referrer spam attempts. It requires no upfront configuration and no mod-rewrite. It is like the strong silent bouncer at your favorite club and it works well.
This plugin implements spam filtering using Naive Bayesian classifier. This plugin will automatically classify new comments as legitimate or spam based on past decision done by you. It requires modification to a few WordPress core files so it may not be suited for completely new users.
Selectively close comments on your blog to help reduce spam. It comes with an administration page and is fully configurable.
This plugin provides protection against email harvesters by taking email addresses and mailto links on your blog and converting their text to numeric character references.
ImgProtectr will take any text you pass to the imgProtectr() template tag and put an image with the specified text in itís place. This is primarily intended for use in templates where you would have an email address displayed.
Did You Pass Math?
To make things very easy for a legitimate commenter, this small plugin asks the person making the comment to answer a simple math question. This is intended to prove they are a human being and not some kind of robot.
This plugin displays an image with some random text that the commenter has to enter in order for their comment to go through. This should cut down on any bots out there from spamming your comments area and perhaps remove the need for comment moderation.
Hardened Trackback generates single-use, expiring trackback URLs with AJAX. Whenever someone tries to send a trackback without a valid key it will be ignored.