Akismet

Akismet ships with new installations of WordPress. You'll find it already installed when you first set up your site. Akismet is a valuable anti-spam plugin that guards against the pervasive problem of comment spam. It is very effective at preventing spam that it blocks about 99 percent of all spam comments. You'll need to sign up at http://akismet.com/ to get a free key to run the program. There are paid versions, but the basic functionality is available with the free offering.