I am sure that you have heard of Captcha by now, if you haven’t then I guess you are not in the technology field. Captcha’s are those annoying images with text that you have to enter before registering for most forums and posting comments in some blogs.
Captcha’s work because they can’t be read by computers so a human is required. Here’s a website thats using this necessary evil  and putting it to a good use. Basically what it does is uses captcha’s to convert scanned books to text.
Why do we need to do that, Can’t we just use OCR (Optical Character Recognition) to convert the books to text you may ask? Well its like this, OCR is not perfect and it has a lot of trouble with certain kinds of words e.g. clot might get read as dot etc. So what reCAPTCHA does is that it uses sending words that cannot be read by computers in the form of CAPTCHAs for humans to decipher and then saves the converted text. So with a lot of usage books that were previously inaccessible would slowly become accessible.
If I used captcha’s I would definitely use this as it is a good cause but I don’t use Captcha’s as they don’t work with text browsers. After I had to spend 20 mins trying to download a file that was protected with a captcha on a server to which I only had ssh access (I got around it by saving the image file to the computer, copying it to my local system, viewing it and then entering the code… ) I decided that I will not use it on any of my systems/sites.
Anyways, Check out reCAPTCHA and if you have to use Captcha’s use this one instead.