Spam Karma 2 (SK2) is an anti-spam plugin for the WordPress blogging platform. It is meant to stop all forms of automated Blog spam effortlessly, while remaining as unobtrusive as possible to regular commenters. Spam Karma 2 is the proud successor to Spam Karma, with whom it shares most of the development ideas, but absolutely none of the code. You can imagine the popularity of this plugin by looking at over 1900 comments under the post.

Pricing: Free
Requirements: WordPress 1.5.1+
Source: http://unknowngenius.com/blog/wordpress/spam-karma/