I just heard of reCAPTCHA. This is a very interesting spin on CAPTCHAs. Accoring to their web site, 60 million captchas are solved every day. reCAPTCHA uses part of the effort to "scan" books.
A reCAPTCHA captcha contains a word that OCR didn't find a match when scanning books from the Internet Archive. In addition to the word that is not recognized, a successfully scanned word is displayed. The captcha is considered successfully entered if the known word is entered, and then the answer for the unknown word is added to a database. If enough people enter an unknown word with the same answer, then the probability of the answer being correct goes up.
There are several plugins for blogging systems, as well as a PHP api.