From here:

Thanks for the pingback. It was appreciated and I have responded in turn! I agree that reCAPTCHA is a pretty cool web application and it certainly beats all of the current spam tools except for maybe Akismet. My only gripe with them as I mentioned is that I would need an enterprise key which I don’t really want to buy obviously. Therefore in terms of any plugins with captcha (really the only way to go) is with the reCAPTCHA plugin. Highly Recommended! 5 Stars!