Zero Spam makes blocking spam comments and registrations easy.
Prevents automated SPAM by requiring visitors to answer a question of your choosing (plaintext or regex).
O RLY Comment Spam Search creates a quick link to make sure comments aren't spam.
Spam BLIP stops comment spam before it is posted, using DNS blacklists, existing comments marked as spam, and user defined lists.
Add an easy-to-use spam filter to your comment form.
Convatic is a revolutionary social commenting system for websites. This plugin overlays and plays nicely with themes and other plugins. Convatic promo
This plugin will stop comments spam entirely, for free. It requires all users to connect their verified RubyID account before they can post a comment.
WP True Typed is an anti-spam plugin to protect your site from comment spam.