Plug in for Authentication?

I have viewed/looked into all the plug-ins I could find for WP3.

I am seeking a word challenging test for people who join my blog, like other sites have, i.e., when a little box shows letters over top of a textured background and the user must type in what they see.

Is there anything like this for WP3? I think a couple of member signups may be from auto/search engine hackers and I wish to stop that from happening.

Thank you in advance!