In short, reCAPTCHA is a standard CAPTCHA with the source for the images coming from old hard copy documents that need to be digitized.
Why is this truly genius? It solves two problems in one shot and everybody wins. On one side there is a group of people that are trying to digitize old printed text, much of which is too damaged to use OCR scanning technology.
On the other side, images for CAPTCHA need to be generated. I know this is an automated task and no longer an very difficult issue, but nonetheless, new images are needed.
So we've got a source of images, and a need for images. Apply a bit of technology to take care of the details and you've got a pretty good solution.
Now books are digitized at a faster pace, and the cost of all the man hours is saved. According to the article over 100 million CAPTCHA's of all types are handled annually. Imagine the savings! So far forty thousand sites have the free reCAPTCHA tool. I'd love to see reCAPTCHA's own stats on how many words have been process in the year they've been live.
Simple and everybody wins. Genius.
I've installed the reCAPTCHA Wordpress plugin on this site. Check it out below in the comments.