Digitizing words, one at a time. This is a great way to mix crowdsourcing and security. Captcha (for Completely Automated Public Turing test to tell Computers and Humans Apart) is used to ensure (with varying degrees of success) that only real humans are logging onto websites. reCaptcha uses words that have failed optical character recognition in various projects that are digitizing books not otherwise available.
Check out the site, and if you have a compatible web site, consider using it for validating your users.