Defensio is a spam filtering web service that you can use to protect your blog or web application from comment spam.
Features: Spam quarantine sorted by spaminess. OpenID support when used in conjunction with the WP-OpenID plugin. Background color applied to spam comments to increase ease-of-identification. Spam comments collapsed (and restorable) to allow for easy and efficient navigation. Very obvious spam can be hidden from view. Performance statistics provided to show how Defensio is doing on your blog.