I just ran into this, and haven’t time at the moment to integrate it into EE, but would like to share the word here.
This is from CMU (Carnagie Mellon University), and from the inventors of CAPTCHA.
It actually puts the captcha to use, in an interesting way - read about that here.
http://recaptcha.net/learnmore.html
And it includes a very nice audible option as well as the visual one. Attractive, too.
http://recaptcha.net/captcha.html
With New Year regards,
Clive