During the recent book update, we needed to make some room for the new WordPress-3.1 content. The book is already over 400 pages and growing. So we have to make some hard decisions about which content is useful but maybe not needed in the book. And, as useful as long lists of anti-spam plugins might be, moving them from the book to the blog seems like a good way to free up some room while keeping the information available. So without further ado, here is a quick list of 15 anti-spam plugins to help you run a more user-friendly, hassle-free comment system.

Akismet

The best anti-spam plugin for WordPress. Bundled with WordPress, Akismet requires a registration key, but is easy to setup and provides excellent “set-it-and-forget-it” spam protection for WordPress.

bcSpamBlock

JavaScript-based anti-spam plugin that uses JavaScript to filter out spam quietly and discretely. Users without JavaScript must prove their legitimacy via copy-&-paste CAPTCHA exercise.

Comment Spam Stopper

Blue Anvil’s anti-spam plugin is CAPTCHA-based and includes JavaScript validation to ensure that required fields in the comment form have been populated with data. To save time, the CAPTCHA field is not displayed when logged into Admin.

Comment Spam Trap

Delivers a double death blow by adding a hidden spam field and an identical but required CAPTCHA field. This simple logic tricks mortal spam bots into revealing themselves and getting blocked. Also blocks trackback spam and optionally sends email notifications of anything it blocks.

Cookies for Comments

Takes a different approach by adding a randomly generated stylesheet URL to your theme. When that URL is requested by the browser, a cookie is set that is required for the visitor/user to leave a comment. The plugin homepage is kinda thin, so scan the WordPress Forum to gain more insight about this remarkable plugin.

Did You Pass Math?

Requires the user to solve a simple math problem, like “what’s 1+2?” If they can’t do it, they’re considered a bot and the comment is blocked. Simple yet effective anti-spam plugin with nothing to configure – just set and forget.

JSSpamBlock

Uses JavaScript to filter out spam bots and their filthy comments. Legitimate users prove their identity by entering a given number. Provides fallback for non-JavaScript visitors.

Peter’s Custom Anti-Spam

A full-featured CAPTCHA-based anti-spam plugin for WordPress. Requires all commentators to identify a random word before comment submission. Words are displayed as images and are completely customizable. Features: random font display, no cookies required, no JavaScript required, auto-generated audio for visually impaired users, selective blocking of trackbacks and pingbacks, and much more. First choice for full-flavored CAPTCHA plugin.

reCAPTCHA Plugin

Displays words from old books that users must correctly interpret. Uses the popular reCAPTCHA service that is used on popular sites such as Twitter, Facebook, and StumbleUpon. Upside: use of this service helps to digitize old books. Downside: requires a key to work.

Referrer Bouncer

Referrer Bouncer provides powerful protection against referrer spam. Easy to use and requires no configuration. As it says, “It is like the strong silent bouncer at your favorite club.” The instant cure for the referrer spam that ails you.

Simple Trackback Validation

Solid protection against trackback spam. Trackback validation is done with an IP/referrer check and by checking the trackback page for your URL. Bottom line: an excellent solution for stopping trackback spam. Includes Settings Page for easy configuration, including the option to delete or spam blocked trackbacks.

Spam Free

Spam Free is an “extremely powerful anti-spam plugin for WordPress that eliminates comment spam, including trackback and pingback spam.” Spam Free has many features, including no CAPTCHA required for site visitors, a spam-free contact form, and dashboard counter with blocked spam count.

Word Verify

CAPTCHA-based anti-spam plugin that requires the user to enter a simple word in plain text (rather than an image). This makes it much easier for users to get it right the first time, while filtering out lots of automated spam. This plugin is probably best for smaller sites and blogs, as they aren’t generally targeted by the heavier OCR-capable spambots. Or so the thinking goes. Includes Settings Page for basic configuration.

WP-HashCash

CAPTCHA-based anti-spam plugin that claims to be 100% effective at blocking all spam and no real comments. Also blocks most pingback & trackback spam. Features Settings Page for statistics and configuration. And a huge bonus, WP-HashCash is “100% standards compliant XHTML 1.1 and works with both jQuery and Prototype.”

Note that the book now contains an abbreviated version of this list, along with sidebar mentions of some of the other plugins sprinkled throughout the various chapters. Going through this list again for the post, it was great seeing the wide variety of sites and personalities involved in keeping WordPress spam-free. If you know of any good anti-spam plugins that we missed, feel free to share them in the comments.