I get that people are here to hate on Google, but I am just here to say that reCAPTCHA albeit acquired, is an absolutely brilliant idea. The kind that solves two (three? if you count tracking) problems so elegantly
The people who created the initial version that got bought went on to create duolingo, with a similar goal of getting people to produce translations of text.
Multi-purpose trojan horse. Not only will it look beautiful in your city but you can use it as scaffolding to repair tall buildings or children in your community could use it as a play gym.