Read this article on BBC News about the technology of CAPTCHA (or Completely Automated Turing Test To Tell Computers and Humans Apart), which was originally designed to separate humans from computer bots.
Interestingly, besides stopping automated bots from spamming, CAPTCHA also serves a higher purpose of helping university researchers preserve old books and manuscripts.
These old books and manuscripts are initally scanned by OCR software, but because of the poor quality of the original documents, the OCR software is not able to decipher the words. So, what the Carnegie Mellon University researchers (they were the creators of CAPTCHA) did was to make use of these words to be used as CAPTCHAs (or in this case, they are known as reCAPTCHAs). And when you key in the correct words, you are essentially helping the Carnegie Mellon team to digitise another word from an old book or manuscript.
So, the next time you have to type in one of these CAPTCHAs, don’t dread it as you’re helping in the digitization efforts of researchers of old books and manuscripts.