Akismet is a hosted web service that saves you time by automatically detecting comment and trackback spam. It's hosted on our servers, but we give you access to it through plugins and our API.
How does it work?
How does it work?
Each time a new comment, trackback, or pingback is added to your site it's submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down. As a result, you don't have to waste your time sorting through and deleting spammy comments from your blog.
Link: http://www.akismet.com/