Reading by numbers

I'm a big fan of games that effectively use crowdsourced input as a solution to a very difficult problem. There are a number of examples like Google Image Labeler, and even the Guardian's attempt to keep their government transparent, by investigating expenses that "merit further investigation". However reCAPTCHA is a particularly effective one created by Carnegie Mellon Professor Luis von Ahn, which harnesses a reported 200 million solutions everyday. reCAPTCHA works by presenting a user with two words: one control, and one which could not be read by a computer. A human then enters both words, usually in an attempt to finalize a purchase at Ticketmaster, and if the control word is correct, it is assumed that the unknown word is also correct. The beautiful result: humans reading books, one word at a time, unlocking digital content along the way.