This falls squarely in the category of Who Knew?
You know those reCaptcha things you see on web sites? Where you have to type two words to validate whatever it is that needs validating? Annoying, aren't they?
Did you know they actually serve a purpose other than to piss you off?
One of the words is a plain old 'are you a human' test, as one would expect. But the other word is a piece of scanned text from a document that OCR software was unable to recognize. By taking a stab at it, you're helping to digitize old books, newspaper articles, and the like.
Serious! Check it out here, and by the way, that reCaptcha box at the top of that page is live, so digitize some words while you're there, wouldya? I can't do it all myself, you know.