reCAPTCHA

A CAPTCHA system originally developed at Carnegie Mellon University (and later acquired by Google) that repurposes human effort spent proving they are not robots into useful work such as digitising scanned books or labelling images for machine learning. reCAPTCHA is a frequently cited example of "crowd-harvested" work because it collects micro-contributions from millions of users without their needing to volunteer. From an accessibility perspective reCAPTCHA is controversial: visual and audio challenges have historically created serious barriers for screen reader users, people with low vision, and those with hearing impairments, though the modern reCAPTCHA v3 uses behavioural scoring that avoids most visible challenges.