WordPress.org

This plugin works by adding a textarea field to the comment form that's hidden using a CSS style. Since bots don't generally detect CSS like this, they proceed to fill out the field like any other field. This implies that they aren't a human (or they wouldn't even see the field with CSS hiding it) being using a browser, in which case the plugin marks the comment as spam. This seems to catch the vast majority of spam with very few false results and doesn't require contacting a third party service like most spam plugins.

If you like this plugin, please consider making a small donation to support its development here: http://bit.ly/K1jRVo