This plugin is no longer maintained.
About time I had TuringTest listed as being for v2.* eh? The same plugin exists here, but it says for v1.8 and v1.9. This is a very simple "challenge and answer" where you get to ask your commenters (or message form users or registrants) a question they must answer correctly. If they don't you will be able to give them a hint and let them try again.
The link to this extension is broken.