Anti-spam Pro plugin blocks spam in comments.
Anti-spam Pro is extended version of free Anti-spam plugin – http://wordpress.org/plugins/anti-spam/
Anti-spam plugin blocks spam in comments automatically, invisibly for users and for admins.
No captcha, because spam is not users’ problem. No moderation queues, because spam is not administrators’ problem.
More info at – http://web-profile.com.ua/wordpress/plugins/anti-spam-pro/