CAPTCHA

CAPTCHAs can be used by MediaWiki wikis to prevent spam. Several implementations currently exist in extensions. Extension:ConfirmEdit is the base framework upon which other CAPTCHA extensions are generally built.

Feature idea
Write a version of reCAPTCHA that uses document images that have been processed by MediaWiki's ProofreadPage extension for WikiSource. In other words, a CAPTCHA that feeds data to ProofreadPage to augment its OCR processing. You might build on existing code.