+1 for akismet
http://wordpress.org/extend/plugins/cookies-for-comments/
Is also amazing. Akismet is great at catching spam, but adding cookies for comments is awesome. It stops a lot of the auto-posted spam. (Which most of it is)
Cookies for comments will stop the comment before it even gets to you. Akismet was catching 50 spams a day for me. But once I added CFC, I only get maybe 1 comment a week for akismet to deal with. The two combined are great