Akismet is a wordpress plugin for reducing spam comments. It works by sending the info left by the commentor, to the Akismet servers, which checks whether the comment is spam or not. And it’s completely free for personal blogs.
Well, as far as i have noticed, every wordpress installation is pre-installed with akismet, You just need to configure it, which would hardly take a minute.
The Process is simple, just sign up at Akismet.com, and you’ll get an API key, once you have got it, goto your dashboard, activate Akismet, and enter your key, which was mailed to you.