It's on the plugins page, but I think I never made a forum post about it, so here's a forum post about http://wonderwinds.com/hackblog.php/2007/01/07/turing_test_plugin - my simple turing test plugin.

It lets you set a question that the commenter or message form user will have to answer before they can comment or send a message. You, of course, set the answer. You also set the message they see if they give the wrong answer. In v0.3 you also get to set a second "question & answer & text if wrong" to use on those who try to register on your blog. Hoo Ray eh?