I heard a very interesting podcast interviewing Luis von Ahn, one of the inventors of the CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart). These are the images of distorted letters you have to type. They let the service know that you are a real person and not a program.
von Ahn has now created the reCAPTCHA. Instead of using random letters, he is using parts of scanned materials that need to be digitized. Ultimately, solving a CAPTCHA also helps digitize a book!
So, how do you know you typed the correct letters if the system hasn’t already decoded it? You are given two words to decode — one is new and one has been decoded by several other people. If you decode the second one correctly (as the others before you did), then there is higher confidence that you decoded the first one right. The new word is sent to multiple people; when there is agreement, then it is assumed the word has been correctly decoded.
So, remember — if you’re frustrated with those CAPTCHAs — you may be performing a public service by digitizing part of a book!