Akismet

Automattic Kismet, is a spam filtering service created by Automattic, the corporation which employs most of the main developers of the weblog platform WordPress. Akismet attempts to filter link spam from blog comments and spam TrackBack pings. The filter works by combining information about spam captured on all participating blogs, and then using those spam rules to block future spam. Akismet has captured over 200 million spam comments and pings as of 17 September 2006.

Akismet was originally developed to integrate with a plugin for WordPress. It is now included by default in all WordPress builds since version 2.0 and activated in all WordPress.com-hosted blogs. A public Akismet API has resulted in an Automattic sponsored plugin for Movable Type, and third-party plugins for other platforms including blojsom, Geeklog, Nucleus, Drupal and Simple Machines Forum. Released under the GNU General Public License, the Akismet plugin is free software.

The Akismet service is free for personal use. To use Akismet with most blogs, it is necessary to obtain a WordPress.com API key. Bloggers with a Yahoo! hosting account get a free “pro-blogger” Akismet service (which costs non-Yahoo! users $5/month) due to a deal between the two companies.

Source: Wiki