This plugin is no longer maintained.
Turing Test lets you ask your commenters and message form users a question that they have to answer correctly before they can comment or send a message. As the admin you get to decide the question, the answer, and the message someone will see if they don't give the correct answer. You also get to decide who, if anyone, will get the answer automagically filled in for them. You can pick "no one", "registered members", or "previous commenters".