Akismet filters out comment and track-back spam for you . The best part it since it blocks spam you will not receive all the emails ( if you have opted for ) asking you to moderate the comments.I am using the FREE version of the same. Tested and working for Wordpress 3.8 release.
Click here to go to the Plugin page http://wordpress.org/plugins/akismet/
Click here to go to the Plugin page http://wordpress.org/plugins/akismet/