Perhaps you have been demanded to input some words displayed on screen. These letters are a little difficult to read, sometimes dirty or blurred.
This is a solution to prove you are a human, not a computer. Current computer program cannot identify such picture as a meaningful word, different from human. Therefore, the website can be protected from an attack of programs with malice.
However, the process of your input is not only for the security check, but also utilized for an amazing work. This program named reCAPTCHA, developed by Luis von Ahn and his colleagues, delivers enormous words hardly read by computer and gather the inputted data. It helps the process of optimal character recognition (OCR) of ancient books.
Luis von Ahn explains the mechanism in a TED conference with humor.
YouTube: Luis von Ahn: Massive-scale online collaboration
Therefore, all we are contributors to digitize lots legacies, unconsciously. This idea is splendid in terms of combining two quite different concepts: certification procedure and shared working.
To be honest, I knew this fact previously. Nonetheless, his presentation was so impressive. He has also begun an language education service, Duolingo. You can train your language skill for free via the website. In the training, you are involved in some translation tasks. Of course, it will be beneficial for others who require some materials written in their native language.
This idea is also good. Most training process include some kind of waste. Many people pump the pedals of a bike only to reduce the body weight. It has been realized to connect the pedal they kick into an electric generator. Computer and the internet have made it easier to join lots of people into an integrated work. As Ahn says, we would be able to accomplish a work greater than reaching the moon, some day.