Started thinking about captchas, all "build a better mousetrap." Except I don't want the sort of people who want a better captcha at my door.
Came across
reCAPTCHA, where CMU gives you two words (one known, one unknown), and in turn uses you to fill the gaps in the OCR software they have processing old books.
I feel as though it might be 'nifty social work' at the cost of 'extreme security.' But it's free.