SpamTask is a new anti-spam plugin for WordPress, and catches all spam robots with 100 % certainty. It's the most advanced spam filter for blogs in the world, and exactly what you need if you're tired of spam comments getting through on your blog.
You can start using it just by downloading and activating it, as your blog will have 10 test points every month. After that, it only costs at highest €0.01 per message, with a cap limit on €7.5 per blog, every month.
For more information: http://wordpress.org/extend/plugins/spamtask/