reCAPTCHA
I'm sure that you have seen a captcha somewhere on the web. A captcha is an image that you have to type the contents of to continue on in a form. We use them at work from time to time. I've been interested in but never used the reCAPTCHA system. Recently purchased by google, I get the feeling that they will see alot more use in the coming months.
reCAPTCHA is a system that solves two problems in one. Optical Character Recognition has always had the issue of hard to read words. Computers simply can not read quite like humans. This is precisely why captchas work so well. To defeat a captcha and spread spam, a bot would have to be great at OCR. reCAPTCHA takes scans of old books and presents them to users. One of the words presented is known and the other is unknown. When a certain number of people answer the same on the unknown word, it is considered known. Using the reCAPTCHA system, we are digitising books one word at a time.